Category: Towing & trailers

Quick answer

Most trailer light faults come down to four things: a poor earth, corroded plug or socket pins, a blown globe, or a blown fuse or faulty module on the vehicle side. Start by testing the vehicle’s socket with a trailer tester. If the socket works, the fault is on the trailer, and the earth is the first thing to check. Lights that glow dimly, flash together or work the “wrong way” almost always point to a bad earth.

Is this a DIY job?

• Difficulty: Easy
• Time: 15 to 60 minutes
• Tools: trailer circuit tester (or a helper), multimeter or test light, wire brush, contact cleaner, screwdriver, spare globes
• Skills: basic electrical checks

Step by step

1. Test the vehicle side first. Plug a trailer circuit tester into the vehicle’s socket, or use a test light between each pin and pin 3 (earth). Check tail lights, left and right indicators, stop lights and reversing lights. This tells you which half has the fault.
2. If the vehicle socket fails: – Check the towbar or trailer fuses (many vehicles have dedicated ones; see the owner’s manual or the towbar harness instructions). – Look inside the socket for green corrosion or bent pins. Clean with contact cleaner and a small brush. – Many modern vehicles feed the socket through a trailer module. If the module has no power or has been damaged, the socket won’t work. (see Do I need a vehicle-specific towbar wiring harness?)
3. If the vehicle socket works, check the trailer plug. Open it and check every wire is tight and there’s no corrosion. Cut back and re-terminate any wire that’s green or brittle. (see How do I wire a 7-pin flat or 7-pin round trailer plug?)
4. Check the trailer earth. Follow the white earth wire from the plug to where it bolts to the trailer chassis. Remove the bolt, clean the terminal and the chassis back to bare metal, refit and protect it. Many trailers also rely on each lamp’s mounting for earth: check those too.
5. Check the globes. Swap in new globes or test them. A blackened or broken filament means it’s blown. LED lamps can’t usually be repaired; replace the lamp.
6. Check the lamp sockets. Corrosion in a lamp holder causes flickering. Clean it, and use a little dielectric grease to keep moisture out.
7. Look for damaged cable. Check the cable along the drawbar and under the trailer for cuts, crushing or rubbing, especially where it passes through holes without grommets.
8. Retest everything before you tow, including brakes if fitted.

Specs table

Symptom and likely cause

Symptom	Likely cause
Nothing works	Vehicle fuse or trailer module, plug not fully in, broken earth
Lights dim, or glow when other lights are on	Poor earth (most common)
Indicator lights up the tail lights, or both sides flash	Poor earth
One function missing on both sides	That pin or wire at the plug or socket
One lamp out	Globe, lamp socket, or that lamp’s earth
Works when wiggling the plug	Corroded pins or loose wire in plug or socket
Indicators flash fast with trailer on	Trailer globe out, or LED trailer lamps on an older flasher
Trailer lights fine, brakes weak	Earth, pin 5 connection, or brake controller setting

Pin reference (7-pin flat and 7-pin large round): 1 left indicator (yellow), 2 reversing (black), 3 earth (white), 4 right indicator (green), 5 electric brakes (blue), 6 stop (red), 7 tail (brown).

What you’ll need

• trailer circuit tester
• 12V test light
• electrical contact cleaner
• dielectric grease
• ring terminals and adhesive heat shrink
• LED trailer lamps or trailer globes
• 7 pin flat trailer plug (if the plug is badly corroded)

When to call an auto electrician

• The vehicle socket doesn’t work and the fuses are fine.
• The trailer module or towbar harness has failed, or the vehicle shows trailer or lamp warnings.
• The trailer cable needs replacing along its length, or the trailer has electric brakes that aren’t working properly.
• You can’t find the fault after checking the earth and plug.

Related questions

Q: Why do my trailer lights flicker? A: Usually a poor earth or corroded pins. Clean the earth point back to bare metal and clean the plug and socket.

Q: Why does my car’s indicator flash fast with the trailer connected? A: A trailer indicator globe is likely blown. If the trailer has LED lamps and the vehicle is older, the flasher may not recognise the low load.

Q: Can I use the towball for the trailer earth? A: No. The towball and coupling aren’t a reliable electrical path, and the trailer rules require a dedicated earth return wire between the trailer and the vehicle. Use the white earth wire through the plug.

Q: Do LED trailer lights need special wiring? A: Usually not, but they’re sensitive to poor earths, and some vehicles need a trailer module to recognise them.

Sources

1. ExplorOz, “Trailer wiring diagrams” – pin functions and colours. https://www.exploroz.com/vehicle/electrics/trailerwiringdiagrams.aspx (checked October 2026)
2. Department of Infrastructure, Vehicle Standards Bulletin 1 (Revision 6, August 2026) – trailers up to 3,500 kg ATM use a 7-pin or 12-pin plug to AS 4177.5; 7-pin functions and colours; an earth return wire between trailer and tow vehicle must be provided; required trailer lamps. https://www.infrastructure.gov.au/sites/default/files/documents/vsb1-revision-6-trailers-with-an-aggregate-trailer-mass-of-4.5-tonnes-or-less-august-2026.pdf (checked October 2026)

Quick answer

On most modern vehicles, yes, it’s the safer choice. Many current cars and utes monitor the current in each rear light circuit and use it to detect bulbs and trailers. Splicing trailer wiring into the tail-light loom can trigger warnings, upset trailer detection features and risk damaging the body control module. A vehicle-specific harness plugs into the factory connectors without cutting, and usually includes a trailer module that powers the trailer lights separately. Older vehicles without electronic lamp monitoring can often use a universal kit.

Is this a DIY job?

• Difficulty: Medium (plug-in harness); Pro (vehicles needing coding or a module install behind trim)
• Time: 1 to 2 hours for a plug-in harness
• Tools: trim tools, socket set, multimeter or trailer tester, cable ties
• Skills: removing boot or tray trim, routing wiring neatly

How the options differ

Option	How it works	Suits
Vehicle-specific (plug-and-play) harness	Plugs into factory tail-light connectors with matching plugs; a module reads the signals and drives the trailer lights from its own fused battery feed	Most vehicles with CAN-bus or monitored lighting
Universal kit with isolating module	Taps into lamp wires; a module isolates the trailer load from the vehicle circuits	Some vehicles, where the harness maker says it’s suitable
Direct splice	Trailer wires joined straight to tail-light wires	Older vehicles without lamp monitoring only

Step by step

1. Check what your vehicle needs. Look in the owner’s manual, the towbar maker’s fitting guide or ask the harness supplier. Some vehicles also need a dealer or scan-tool setting to enable trailer mode.
2. Choose the right harness. Match make, model, year and body type. Check whether it includes a 7-pin flat or 12-pin socket, and whether it has an electric brake signal output.
3. Disconnect the battery negative if the instructions say to.
4. Remove the trim to reach the tail-light connectors. Keep clips and screws together.
5. Plug the harness in-line at each tail-light connector as instructed. Don’t cut factory wiring.
6. Mount the module where the instructions say, away from water and heat.
7. Run the power feed from the battery, fused near the battery, if the harness needs one. Route it away from exhaust and sharp edges, using grommets through panels.
8. Fit the socket at the towbar with its cover and seal.
9. Test every function with a trailer tester: tail, both indicators, stop, reverse, and brake output if fitted.
10. Check for warnings on the dash. Then refit the trim.

Specs table

Signs a vehicle needs a module or plug-in harness

Sign	Why it matters
Bulb or lamp-failure warnings on the dash	The vehicle monitors lamp current
LED tail lights from the factory	Very low current; often monitored
Trailer stability or trailer-detection features	May rely on detecting a connected trailer
Owner’s manual or towbar guide calls for a specific harness	Follow the maker

What you’ll need

• towbar wiring harness (vehicle-specific)
• 7 pin flat trailer socket or 12 pin flat trailer socket (if not included)
• trailer circuit tester
• inline fuse holder (if the harness needs a battery feed and doesn’t include one)
• split loom tube and cable ties

When to call an auto electrician

• No plug-in harness exists for your vehicle.
• The vehicle needs trailer mode enabled or a module coded.
• You also need an electric brake controller, a 12-pin auxiliary circuit or a DC-DC charger.
• Warnings appear after fitting, or the trailer lights behave oddly.

Related questions

Q: Can I just splice into my tail lights? A: On older vehicles without lamp monitoring, a careful splice can work. On most modern vehicles it risks warnings, faults and problems with trailer detection, so a plug-in harness is the safer choice.

Q: Does a towbar harness run electric trailer brakes? A: A lighting harness alone doesn’t. Electric brakes need a brake controller wired to pin 5. (see How do I install an electric brake controller?)

Q: Will a towbar harness affect my warranty? A: A plug-in harness that doesn’t cut factory wiring is less likely to cause disputes than a spliced install. If you’re concerned, ask your dealer before fitting.

Q: Why do my trailer lights work but the car shows a warning? A: The vehicle may be detecting an unexpected load on its lamp circuits. That usually means a splice or an incompatible module. Have it checked.

Sources

1. EV Parts 4×4, “GWM Cannon trailer wiring: dedicated vs universal” – example of a body control module monitoring lamp current; risks of splicing; plug-in harness with module (vendor article, one vehicle example). https://evparts4x4.com/blogs/featured-articles/gwm-cannon-trailer-wiring-dedicated-vs-universal (checked October 2026)
2. Department of Infrastructure, VSB 1 Revision 5 – trailer connectors to AS 2513 or AS 4177.5; pin 5 of a 7-pin connector recommended for service brakes only. https://www.infrastructure.gov.au/sites/default/files/documents/vehicle-standards-bulletin-1-revision5-trailers-with-an-aggregate-trailer-mass-of-4-5-tonnes-or-less-june2009-pdf.pdf (checked October 2026)
3. ARB, Hayman Reese CAN ECU module for towing systems – modern vehicles use CAN-bus networks, and a module manages trailer light and brake signals without upsetting vehicle systems or triggering dash warnings (manufacturer product page). https://www.arb.com.au/product/twle70029-hayman-reese-can-ecu-module-for-towing-systems (checked October 2026)

Quick answer

The Australian 7-pin flat and 7-pin large round carry the same seven circuits: indicators, tail lights, stop lights, reversing lights, electric brakes and earth. They’re just different shapes. A 12-pin flat carries those same seven circuits plus five more, typically a permanent 12V feed (for a caravan battery or fridge), an ignition-switched feed, a second earth, rear fog lights and a spare. Choose the connector that matches your trailer, or use an adaptor that carries every circuit you need.

Is this a DIY job?

• Difficulty: Easy to identify; Medium to change connectors; Pro for charging circuits on modern vehicles
• Time: 5 minutes to identify; 30 to 90 minutes to rewire a connector
• Tools: trailer circuit tester, owner’s manual, screwdriver
• Skills: recognising the connector and checking each circuit

Which one do you have?

1. 7-pin flat: a flat, rectangular plug. The most common connector on Australian light trailers, boat trailers and many caravans.
2. 7-pin large round: a large round plug, common on older trailers, some boat trailers and heavier equipment.
3. 12-pin flat: a larger flat plug, common on caravans and campers that need power from the tow vehicle as well as lights and brakes.
4. 7-pin small round (European): a smaller round plug used on imported European vehicles and trailers. It’s wired to a different standard and isn’t interchangeable with the Australian 7-pin round without the correct adaptor.

Step by step: matching a vehicle and trailer

1. Identify both connectors. Look at the socket on the vehicle and the plug on the trailer.
2. List what the trailer needs. Lights only? Electric brakes? Reversing lights? A caravan battery or fridge charged while towing?
3. If the connectors differ, use an adaptor that carries every circuit you need, especially electric brakes (pin 5). Secure it so it can’t drop onto the road.
4. Don’t feed power through a 7-pin connector. Pin 2 is for reversing lights. Battery charging or fridge power needs a 12-pin connector or a separate Anderson plug with heavy cable. (see How do I wire an Anderson plug?)
5. Test every function with a trailer tester or a helper before you tow.
6. Consider a 12-pin upgrade if you’re towing a caravan with an onboard battery. The extra pins are only useful if the vehicle side is wired for them, with suitably sized cable and fusing.

Specs table

Pin functions (Australian connectors)

Pin	Function	Colour	7-pin flat	7-pin large round	12-pin flat
1	Left indicator	Yellow	Yes	Yes	Yes
2	Reversing lights	Black	Yes	Yes	Yes
3	Earth	White	Yes	Yes	Yes
4	Right indicator	Green	Yes	Yes	Yes
5	Electric brakes	Blue	Yes	Yes	Yes
6	Stop lights	Red	Yes	Yes	Yes
7	Tail, clearance and side marker lights	Brown	Yes	Yes	Yes
8	Permanent 12V (battery charging, accessories)	Orange	–	–	Yes
9	Ignition-switched 12V or auxiliary	Pink	–	–	Yes
10	Second earth	White	–	–	Yes
11	Rear fog light	Grey	–	–	Yes
12	Spare or auxiliary	Violet	–	–	Yes

Pins 8, 9 and 12 are described slightly differently by different sources, and installers sometimes use them differently. Always test what your vehicle and trailer actually carry before relying on them.

Sockets with or without a reed switch

Trailer sockets are sold in two versions: plain, or with a built-in reed switch. The switch is a small magnetic sensor inside the socket. When the lid opens or a plug goes in, the switch changes state, so the vehicle knows a trailer is connected. Its most common job is to switch off the reversing sensors so they don’t beep at the trailer the whole time you’re reversing.

Reed switch sockets come as normally open or normally closed. Which one you need depends on how the vehicle’s circuit is wired, so check the vehicle or harness instructions before buying. The wrong type will do the opposite of what you want: the sensors stay on while towing and switch off when you’re not.

Connector rules for light trailers: under the national guide for small trailers (VSB 1, Revision 6), 7-pin and 12-pin plugs on trailers up to 3,500 kg ATM must comply with AS 4177.5-2004. It recommends using pin 5 only for service brakes, and a 12-pin connector if you need auxiliary circuits.

What you’ll need

• 7 pin flat to 7 pin round adaptor or 12 pin to 7 pin trailer adaptor
• 12 pin flat trailer socket / 12 pin flat trailer plug
• 12 pin flat socket reed switch if your vehicle has reversing sensors (check normally open or normally closed)
• trailer circuit tester
• trailer plug cover

When to call an auto electrician

• You want a 12-pin socket with working battery-charging and auxiliary circuits on a modern vehicle. Those circuits need heavy cable, fusing at the battery, and often a vehicle-specific harness or DC-DC charger.
• Your vehicle is European and has a small round socket.
• The trailer has electric brakes and you need a brake controller fitted. (see How do I install an electric brake controller?)
• The lights or brakes don’t work properly through the adaptor.

Related questions

Q: Can I plug a 7-pin trailer into a 12-pin socket? A: Yes, with a 12-pin to 7-pin adaptor. Pins 1 to 7 carry the same functions, so lights and brakes work. The extra 12-pin circuits simply aren’t used.

Q: Will a 12-pin plug charge my caravan battery? A: Only if the vehicle’s pin 8 is wired with heavy enough cable from the battery and fused properly. Long, thin cable loses a lot of voltage, so many people use a DC-DC charger in the caravan. (see Why isn’t my caravan battery or fridge charging while towing?)

Q: Is the European 7-pin round the same as the Australian one? A: No. They look similar but are wired differently. Using one with the other without a proper adaptor can make the trailer brake when you switch on your lights.

Q: How do I wire a 7-pin plug? A: See our step-by-step guide with a pin chart. (see How do I wire a 7-pin flat or 7-pin round trailer plug?)

Sources

1. ExplorOz, “Trailer wiring diagrams” – 7-pin flat, 7-pin large round and 12-pin flat pin functions and colours. https://www.exploroz.com/vehicle/electrics/trailerwiringdiagrams.aspx (checked October 2026)
2. Wikipedia, “Trailer connectors in Australia” – 12-pin Type 3 pin allocations, pin 2 caution, ISO 1724 small round differences. https://en.wikipedia.org/wiki/Trailer_connectors_in_Australia (checked October 2026)
3. Department of Infrastructure, VSB 1 Revision 6 (August 2026), section 7.2 – plugs must comply with AS 4177.5-2004 for trailers up to 3,500 kg ATM; 7-pin and 12-pin pin functions and colours; pin 5 for service brakes only. https://www.infrastructure.gov.au/sites/default/files/documents/vsb1-revision-6-trailers-with-an-aggregate-trailer-mass-of-4.5-tonnes-or-less-august-2026.pdf (checked October 2026)
4. Narva, 12-pin flat trailer socket with reed switch for normally open circuits (82074BL) – reed switch deactivates reversing sensors while a trailer is connected. https://www.narva.com.au/products/82074BL (checked October 2026)
5. Narva, 12-pin flat trailer socket with reed switch for normally closed circuits (82073BL) – the normally closed version. https://narva.com.au/products/narva-12-pin-flat-quickfit-trailer-socket-with-reed-switch-for-use-with-normally-closed-circuits-1-pack (checked October 2026)

Quick answer

An electric brake controller mounts in the tow vehicle and needs four connections: a fused heavy-duty power feed from the battery, a solid earth, a signal from the stop-light circuit, and an output wire to pin 5 (blue, electric brakes) at the trailer socket. Mount it where the maker says, set it up with the loaded trailer, and test at low speed. Because trailer brakes are safety-critical and modern vehicles often need a vehicle-specific connection, many owners have it professionally installed.

Is this a DIY job?

• Difficulty: Pro (a confident DIYer with auto electrical experience may do it)
• Time: 2 to 4 hours
• Tools: controller kit and harness, heavy auto cable, crimp tool for large lugs, circuit breaker or fuse, multimeter, trim tools, drill and grommets
• Skills: high-current wiring, routing cable through the firewall, reading the controller’s instructions

Getting it wrong can mean trailer brakes that don’t work, lock up or stay on. If you’re not confident with high-current wiring and your vehicle’s brake light circuit, have it installed by an auto electrician.

Do you need one?

Under the national guide for small trailers (VSB 1), a single-axle trailer up to 750 kg gross trailer mass (GTM) doesn’t need brakes. Above that, brakes are required. Up to 2000 kg GTM, brakes on the wheels of at least one axle are enough, and over-run (mechanical) brakes are allowed. Any brakes other than over-run brakes must be operable from the driver’s seat. Over 2000 kg GTM the brakes must work on all wheels, be operable from the driver’s seat, and apply automatically if the trailer breaks away. Trailers with electric brakes need a controller in the tow vehicle. Check your state’s rules for your trailer.

Types of controller

• Proportional (inertia-sensing): senses how hard the vehicle is slowing and applies matching trailer braking. Generally smoother.
• Time-delayed: applies a preset braking level that ramps up over time once you touch the brake pedal. Simpler, but the trailer may brake too little or too much for the situation.
• Remote-head or in-dash units: the electronics hide behind the dash with only a small control head visible.

Step by step

1. Read the controller’s instructions first. Cable sizes, fuse or circuit breaker rating, and mounting angle all come from the maker. They vary between models.
2. Check for a vehicle-specific harness. Many controllers have plug-in harnesses for particular vehicles, and many modern vehicles monitor the brake light circuit. A plug-in harness, or a towbar harness with a brake signal output, avoids cutting into factory wiring. (see Do I need a vehicle-specific towbar wiring harness?)
3. Disconnect the battery negative. Do this before running power cable.
4. Choose the mounting spot. Within the driver’s reach, where it won’t hit your knees, and clear of airbags and their deployment paths (for example, not on the knee airbag panel). Proportional units may need a specific orientation, or may self-calibrate: follow the maker.
5. Run the power cable. From the battery positive, through a circuit breaker or fuse mounted close to the battery, to the controller. Use the cable size the maker specifies. Pass through the firewall with a grommet and protect the cable in loom tube away from heat and moving parts.
6. Run the earth. Heavy cable to the battery negative or a solid chassis earth point, as the maker specifies. A poor earth is a common cause of weak trailer brakes.
7. Connect the stop-light signal. From the brake light switch circuit, via the vehicle harness or a suitable connection. The controller needs this signal to know you’re braking.
8. Run the brake output to the trailer socket. From the controller output to pin 5 (blue) on the 7-pin or 12-pin socket at the towbar, using the cable size the maker specifies. Fuse or protect it as instructed.
9. Reconnect the battery and check. With the trailer disconnected, the controller should power up. With the trailer connected, many controllers show that they can see the trailer brake magnets.
10. Set up with the loaded trailer. Follow the maker’s setup: some self-calibrate after a few stops, others need you to set gain (output) manually. Start low.
11. Test at low speed. In a safe, empty area, use the manual control to apply trailer brakes alone at walking pace. You should feel the trailer pull back firmly without locking the wheels. Then test normal braking. Adjust gain until the vehicle and trailer stop smoothly together.
12. Check the breakaway system on the trailer separately, following its maker’s test.

Specs table

The four connections

Connection	From / to	Notes
Power	Battery positive, through a circuit breaker or fuse near the battery	Cable size and breaker rating per the controller maker
Earth	Battery negative or solid chassis earth	Heavy cable; clean bare-metal contact
Stop-light signal	Brake light switch circuit	Use a vehicle-specific harness where available
Brake output	Controller to trailer socket pin 5 (blue)	Cable size per the maker; the trailer’s brake earth returns via pin 3

Trailer brake requirements (VSB 1, check your state)

Gross trailer mass	Requirement
Up to 750 kg (single axle)	Brakes not required
Over 750 kg to 2000 kg	Brakes on at least one axle (over-run brakes allowed)
Over 2000 kg	Brakes on all wheels, operable from the driver’s seat, applying automatically if the trailer breaks away

What you’ll need

• electric brake controller
• brake controller harness (vehicle-specific, if available)
• auto cable twin core / heavy duty single core auto cable (size per the controller maker)
• circuit breaker or inline fuse holder
• cable lugs and adhesive heat shrink
• firewall grommet and split loom tube
• 12 pin flat trailer socket or 7 pin flat trailer socket

When to call an auto electrician

• You’re not confident with high-current wiring, firewall routing or crimping large lugs.
• Your vehicle uses CAN-bus lighting or monitors the brake light circuit and there’s no plug-in harness.
• The trailer brakes are weak, grab, or stay on after setup.
• You also need a 12-pin socket, Anderson plug or DC-DC charger for a caravan. It’s often efficient to do these together.

Related questions

Q: Proportional or time-delayed brake controller: which is better? A: Proportional controllers match trailer braking to how hard the vehicle is slowing, which is generally smoother. Time-delayed units are simpler. Choose one that suits your trailer and how you tow, and check it’s compatible with your vehicle.

Q: Does my trailer need a brake controller? A: If it has electric brakes, yes: they won’t work without one. Brake requirements depend on the trailer’s mass, so check VSB 1 and your state’s rules.

Q: Why are my trailer brakes weak? A: Common causes are low gain, a poor earth, thin cable, corroded plug pins, or worn brake magnets. Check the gain setting and the earth first.

Q: Can I move the brake controller between vehicles? A: The controller can often move, but each vehicle needs its own wiring. Plug-in harnesses make this easier.

Sources

1. Department of Infrastructure, VSB 1 Revision 5 – trailer brake requirements by GTM (section 15); brakes on at least one axle up to 2 tonnes GTM; brakes other than over-run operable from the driver’s seat; breakaway requirement; pin 5 for service brakes. https://www.infrastructure.gov.au/sites/default/files/documents/vehicle-standards-bulletin-1-revision5-trailers-with-an-aggregate-trailer-mass-of-4-5-tonnes-or-less-june2009-pdf.pdf (checked October 2026)
2. RACQ, “A guide to trailer brakes” – electric brakes need a control unit in the tow vehicle, with a manual control within the driver’s reach; brakes on at least one axle up to 2000 kg GTM; breakaway systems required over 2000 kg GTM. https://www.racq.com.au/articles/rvs/2018/6/a-guide-to-trailer-brakes (checked October 2026)
3. REDARC, “A beginner caravanner’s review on buying your first electric brake controller” – proportional vs time-delayed; professional installation recommended. https://redarcelectronics.com/blogs/blog/a-beginner-caravanners-review-on-buying-your-first-electric-brake-controller (checked October 2026)

Quick answer

Australian 7-pin flat and 7-pin large round trailer connectors use the same pin numbers for the same jobs: 1 left indicator (yellow), 2 reversing (black), 3 earth (white), 4 right indicator (green), 5 electric brakes (blue), 6 stop lights (red) and 7 tail lights (brown). Wire each core to the terminal with the matching number moulded beside it, make sure the earth is solid, and test every function before you tow.

Is this a DIY job?

• Difficulty: Medium
• Time: 30 to 60 minutes for one plug
• Tools: wire strippers, small screwdriver, side cutters, trailer circuit tester (or a helper), heat shrink and heat gun (optional)
• Skills: stripping and terminating small wires neatly

Replacing the plug on a trailer is a reasonable DIY job. Wiring a new socket into a modern car is different: many current vehicles need a vehicle-specific towbar harness or module, and cutting into factory wiring can cause faults. That part is often best left to an auto electrician.

Plug or socket?

• The socket is on the tow vehicle (at the towbar).
• The plug is on the end of the trailer cable.

The pin numbers and colours are the same on both. These steps describe wiring a trailer plug; the socket is wired the same way.

Step by step

1. Identify the connector. Check whether you have a 7-pin flat, a 7-pin large round, or something else. Don’t confuse the Australian 7-pin large round with the smaller European 7-pin round: they’re wired differently and aren’t interchangeable. If you need extra circuits (battery charging, auxiliary power), a 12-pin flat connector is the usual answer.
2. Disconnect from the vehicle. Unplug the trailer before working on it.
3. Open the plug. Undo the screws and separate the body. Slide the cover and any rubber boot onto the cable before you strip it, or you won’t be able to fit them later.
4. Strip the outer sheath. Remove enough outer sheath to reach every terminal comfortably, without cutting into the inner cores.
5. Strip each core. Strip only as much insulation as the terminal needs. Twist the strands tightly. No stray strands should be able to reach a neighbouring terminal.
6. Match each colour to its number. Each terminal has its number moulded beside it. Use the pin chart and table below. Go by the number, not by where a terminal sits in a picture: viewed from the wire side, the layout is a mirror image of the face.
7. Fasten each core. Insert the twisted strands fully under the terminal screw or into the crimp and tighten firmly. Tug each wire gently to check it’s held.
8. Check the earth. Pin 3 (white) is the earth. On the trailer, run the earth wire to a clean, bare-metal point on the chassis with a ring terminal. Don’t rely on the coupling or towball to carry the earth. A poor earth is the most common cause of dim, flickering or “crossed” trailer lights.
9. Fit the cable clamp. Tighten the strain-relief clamp on the outer sheath, not on the inner cores, so a pull on the cable doesn’t load the terminals.
10. Close the plug. Make sure no wire is pinched, and fit any seals.
11. Test every function. Plug into the vehicle or a trailer circuit tester and check: tail lights, left indicator, right indicator, stop lights, reversing lights (if fitted) and electric brakes (if fitted, using the brake controller’s manual output). Check hazard lights work too.
12. Protect it. Use the plug cover or a holder when the trailer is unhooked, so water and dust stay out of the pins.

Specs table

Australian 7-pin flat and 7-pin large round (AS 4177.5)

Pin	Function	Cable colour
1	Left-hand indicator	Yellow
2	Reversing lights	Black
3	Earth (return)	White
4	Right-hand indicator	Green
5	Electric brakes	Blue
6	Stop lights	Red
7	Tail, clearance and side marker lights	Brown

Notes on these pins

• Pin 2 is for reversing lights. Don’t use it for auxiliary power, a breakaway feed or a pump, even if the trailer has no reversing lights. If you need those circuits, use a 12-pin connector.
• Pin 5 is for electric brakes only. If the trailer has no electric brakes, leave it unused.
• Pin 3 carries the return current for everything else, including the brakes. A good earth matters.
• Colours on older trailers, or on cable made to another code, may differ. Always confirm with a tester before relying on colours.

12-pin flat (for comparison)

Pins 1 to 7 on a 12-pin flat connector follow the same functions and colours as above. Pins 8 to 12 add circuits such as battery charging or auxiliary power, a second earth, rear fog lights and a spare. See (see What’s the difference between 7-pin flat, 7-pin round and 12-pin trailer plugs?).

Trailer brakes (check your state rules)

Braking requirements depend on the trailer’s mass. Under the national guide for small trailers (VSB 1), a single-axle trailer up to 750 kg gross trailer mass (GTM) doesn’t need brakes. Above that, brakes are required. Over-run (mechanical) brakes are only allowed up to 2000 kg GTM. Over 2000 kg GTM, the trailer needs brakes on all wheels, operable from the driver’s seat, that apply automatically and stay on for at least 15 minutes if the trailer breaks away. In practice that means electric brakes, a brake controller and a breakaway system. Check the rules for your trailer with your state road authority.

What you’ll need

• 7 pin flat trailer plug or 7 pin large round trailer plug
• 7 core trailer cable (if replacing the cable)
• trailer plug cover or trailer plug holder
• trailer circuit tester
• ring terminals and adhesive heat shrink
• wire strippers
• towbar wiring harness (vehicle-specific, if fitting a new socket to a modern car)

When to call an auto electrician

• You’re adding a trailer socket to a modern car. Many vehicles use CAN-bus or LED lighting that needs a plug-in harness or isolation module; tapping into tail-light wiring can trigger faults.
• You’re fitting an electric brake controller, breakaway system or 12-pin auxiliary/charging circuit. These carry high current and are safety-critical.
• The lights still don’t work properly after re-wiring, or blow fuses.
• The trailer wiring itself is damaged, corroded or has been repaired many times.

Related questions

Q: Is a 7-pin flat the same wiring as a 7-pin round? A: The Australian 7-pin flat and 7-pin large round use the same pin numbers, functions and colours. The European 7-pin small round is different and shouldn’t be mixed up with them.

Q: Why are my trailer lights flickering or working the wrong way? A: The usual cause is a poor earth, either at the plug or on the trailer. Clean the earth points back to bare metal, check pin 3, and test again. (see Why aren’t my trailer lights working?)

Q: Can I use pin 2 for power to my caravan fridge? A: No. Pin 2 is for reversing lights. Feeding power through it can cause problems with other vehicles and trailers. Use a 12-pin connector or a separate Anderson plug with heavy cable. (see How do I wire an Anderson plug?)

Q: Can I use an adaptor between 7-pin flat and 7-pin round? A: Yes, adaptors are common and the functions line up. Make sure the adaptor carries every circuit you need, especially electric brakes, and that it’s secured so it can’t drop onto the road.

Q: Which way is the plug drawn in wiring diagrams? A: It varies. Some show the face (pin side), some show the wire side. That’s why it’s safest to go by the numbers moulded beside each terminal.

Sources

1. Standards Australia, AS 4177.5 – Caravan and light trailer towing components, Part 5: Electrical connectors (the standard itself is paywalled; pin functions confirmed through the secondary sources below). Reference via Australian Business Licence and Information Service: https://ablis.business.gov.au/VIC/pages/ce32f8e1-d088-45fc-8849-115e51df44ee.aspx (checked October 2026)
2. ExplorOz, “Trailer wiring diagrams” – pin functions and colours for 7-pin flat, 7-pin large round and 12-pin flat. https://www.exploroz.com/vehicle/electrics/trailerwiringdiagrams.aspx (checked October 2026)
3. Wikipedia, “Trailer connectors in Australia” – pin 2 and pin 5 cautions, viewing-direction notes, ISO 1724 differences (cites VSB 1 section 14). https://en.wikipedia.org/wiki/Trailer_connectors_in_Australia (checked October 2026)
4. RACQ, “A guide to trailer brakes” – plain-English summary of brake, breakaway and controller requirements by trailer mass. https://www.racq.com.au/articles/driving-tips/a-guide-to-trailer-brakes (checked October 2026)
5. Department of Infrastructure, Transport, Regional Development, Communications and the Arts, Vehicle Standards Bulletin 1 (VSB 1) Revision 5 – Trailers with an aggregate trailer mass of 4.5 tonnes or less (June 2009) – connectors must comply with AS 2513 or AS 4177.5 (up to 3.5 t ATM); pin 5 for service brakes only; brake requirements by GTM. https://www.infrastructure.gov.au/sites/default/files/documents/vehicle-standards-bulletin-1-revision5-trailers-with-an-aggregate-trailer-mass-of-4-5-tonnes-or-less-june2009-pdf.pdf (checked October 2026)