How to Wire Trailer Lights: Plugs, Pinouts and Sealed Joins
Most trailer light faults are not the globes. They are a green, crusty join behind a mudguard, an earth that relies on a rusty drawbar, or a plug wired to the wrong pin. If you want to wire trailer lights so they keep working through boat ramps, corrugations and wet winters, the job comes down to three things: the right pin assignments, a solid earth and every joint sealed against water.
This guide covers the common Australian trailer plugs (7-pin flat, 7-pin round and 12-pin flat), how to trace and fix a fault, and how to make joins that stay sealed. It is written for box trailers, boat trailers, camper trailers and caravans running 12V lighting.
Quick answer
- Use the Australian colour code: white is the earth, yellow is left indicator, green is right indicator, red is stop, brown is tail and side markers.
- Run a dedicated white earth wire back to the plug. Do not rely on the trailer frame alone.
- Make every splice with a sealed join: a Solderstick connector or a crimp covered with adhesive-lined heat shrink.
- Seal the back of the plug and support the cable so the pins do not carry the strain.
- Test each circuit at the plug with a multimeter or test light before you blame the lights.
Why trailer light wiring fails
Trailer wiring lives in about the worst spot on any vehicle. It gets dunked at the boat ramp, blasted with road grit, shaken on dirt roads and left in the sun for months between trips. Salt water and bare copper are a bad mix: once water gets into a join, corrosion creeps along the strands and the resistance climbs until a light goes dim, flickers or dies.
The usual failure points, roughly in order:
- Earth connections bolted to painted or rusty steel.
- Unsealed splices: twisted wires and tape, or bare crimp connectors.
- Water and dirt inside the plug or socket.
- Cable chafed through where it passes the drawbar, springs or mudguards.
- Wires on the wrong pins after a previous repair.
Fix the earth and seal the joins and most trailer light wiring problems go away.
Australian trailer plug types
Australia uses a few plug styles, and your car and trailer need to match or use an adaptor. The flat plugs are made to Australian Standard AS 4177.5. The round plugs are known as Type 1 (large round) and Type 2 (small round), with the flat plugs as Type 3 in 7-pin and 12-pin versions.
| Plug | Typical use | What it adds |
|---|---|---|
| 7-pin flat | Box trailers, boat trailers, most newer light trailers | Lights, reverse and electric brakes |
| 7-pin small round | Older trailers and some imports | Lights and one auxiliary circuit |
| 7-pin large round | Older caravans and heavier trailers | Lights and a heavier auxiliary feed |
| 12-pin flat | Caravans and camper trailers | The 7-pin flat functions plus battery charge, extra earth and auxiliary circuits |
7-pin flat trailer plug wiring
The 7-pin flat is the plug most people use when they wire trailer lights. The pin assignments for a 7-pin flat plug made to AS 4177.5 are:
| Pin | Function | Wire colour |
|---|---|---|
| 1 | Left-hand indicator | Yellow |
| 2 | Reversing lamps | Black |
| 3 | Earth return | White |
| 4 | Right-hand indicator | Green |
| 5 | Electric brakes | Blue |
| 6 | Stop lamps | Red |
| 7 | Tail, clearance and side marker lamps | Brown |
Pin numbers are moulded into the plug body next to each terminal. Read them off the plug rather than counting positions, because the layout looks different from the front and the back.
12-pin flat and 7-pin round plugs
12-pin flat
Pins 1 to 7 on a 12-pin flat match the 7-pin flat. The extra pins are usually listed as 8 orange (battery charge or winch), 9 pink (auxiliary or battery lead), 10 white (a second earth return), 11 grey (rear fog lamp) and 12 violet (auxiliary). Caravan makers do not all use the auxiliary pins the same way, so check the diagram for your van before you connect a charging feed.
7-pin round
The round plugs use the same colour code, but the pin layout differs from the flat plug and between the large and small round versions. It is good practice to keep pin 5 for electric brakes only, because using it for anything else causes problems when trailers are swapped between cars. Wire a round plug from the diagram supplied with it, and check each circuit with a meter afterwards.
The earth: where most trailer wiring goes wrong
In Australian trailer wiring, the white wire is the earth. Every light needs a clean path back to the vehicle's negative through that white wire.
Many trailers earth each lamp to the frame and then earth the frame through the towbar and coupling. That works when it is new. After a few years of rust, paint and grease, the path through the ball and coupling becomes unreliable, and you get lights that glow faintly or indicators that flash on both sides at once.
The reliable fix is to run the white earth wire all the way from the plug to each lamp, or to a clean earth point on the frame close to the lamps. Where you do earth to the frame, grind back to bare metal, use a ring terminal and a star washer, and paint or grease over the finished joint.
How to wire trailer lights: step by step
1. Isolate and plan
Unplug the trailer from the car. If you are working on the vehicle side socket, disconnect the battery negative first. Check which plug the car has and which lamps are on the trailer.
2. Choose the cable
Multi-core trailer cable keeps the run neat and sheathed. Size the cable for the load and the length of the run; our wire gauge guide explains how. Electric brake and battery charge circuits carry more current than lights and usually need heavier cable, so follow the brake controller or caravan maker's instructions for those.
3. Route and protect the cable
Run the cable inside the drawbar or along the inside of the frame, away from springs, axles and sharp edges. Use grommets wherever it passes through steel and clip it every few hundred millimetres. Leave a drip loop before each lamp so water runs off rather than into the fitting.
4. Wire the plug
Strip only enough insulation to fit the terminal, twist the strands, and clamp them firmly. Fit the cable clamp on the plug body so the sheath takes the pull, not the copper. Follow the pin table above.
5. Make sealed joins
Where the main cable meets each lamp's tails, use a sealed join. That is where most faults start when people wire trailer lights.
6. Test every circuit
Plug in and check left, right, stop, tail and reverse, and brakes if fitted. If something is wrong, check the plug pins with a meter before you pull lamps apart.
Sealing trailer light joins
A trailer join needs to be mechanically strong, electrically sound and sealed at both ends. Solderstick connectors do all three in one step: the solder ring melts into the join, the clear tube shrinks around it, and the coloured adhesive rings seal each end. You can see the solder has flowed through the clear tube before you move on. They carry an IP67 rating (dust tight and able to withstand temporary immersion). Our post on IP ratings explains what that means in practice.
To make the join, strip the wires, slide the connector on, overlap or twist the strands so they sit under the solder ring, then heat with a heat gun until the solder flows and the ends seal. A lighter or torch is a last resort because it can scorch the tube. Let the join cool before you move it.
Lamp tails are usually thin. A red connector suits 0.5 to 1.0mm² wire and a blue connector suits 1.5 to 2.5mm².
Which should you use?
- If you're joining thin lamp tails to trailer cable, use red Solderstick connectors.
- If you're joining heavier brake or auxiliary feeds in the 1.5 to 2.5mm² range, use blue.
- If you're joining cable in the 4 to 6mm² range, such as a caravan charge feed, use yellow.
- If you're sealing the back of a plug, a lamp entry or a bundle of joins, use adhesive-lined heat shrink over the top.
- If the cable is heavier than 6mm² or goes to a battery terminal, use crimped or soldered lugs, not splice connectors.
Trailer lights on boat trailers
Photo: James Coleman on Unsplash
Boat trailers get the hardest life because they go under water. Use submersible LED lamps, keep every join sealed, and seal the back of the plug with adhesive-lined heat shrink. Some owners unplug before launching so the lights are not powered while wet. Rinse the trailer with fresh water after each trip. For more on salt water and wiring, see our guide to marine wiring connections, and for towing setups in general, off-road towing.
Frequently asked questions
What colour is the earth wire on an Australian trailer plug?
White. In Australian trailer wiring the white wire is the earth return on every plug type. If your trailer uses green or black as an earth, it was probably wired to a different standard or repaired with whatever cable was on hand.
Why do my trailer indicators flash on both sides at once?
That is almost always a bad earth. Current from one indicator finds its way back through the other lamps instead of the white wire. Clean the earth points and run a dedicated earth wire to the plug.
Can I use a 7-pin flat to 7-pin round adaptor?
Yes, adaptors are widely sold and work for lights. If the trailer has electric brakes or other auxiliary circuits, check that the adaptor carries those pins through. A permanent matching plug is the more reliable fix for regular towing.
How do I test trailer lights without a second person?
Use a multimeter or test light at the plug, or a trailer tester that plugs into the car's socket. Check each pin for 12V as you operate each switch, then check the white earth back to the car's negative.
Are solder seal connectors waterproof enough for a boat trailer?
Solderstick connectors are rated IP67, which means dust tight and able to withstand temporary immersion. Make sure the solder has flowed and both adhesive rings have sealed, and protect the join from chafing.
Do I need a crimp tool to wire trailer lights?
Not for splices if you use Solderstick connectors, which only need a heat gun. You will still need a screwdriver for the plug terminals and possibly a crimper for ring terminals on earth points.
What to use
For a trailer rewire, the 100PCS Solderstick kit (25 white, 35 red, 30 blue, 10 yellow) covers a full set of lamps with spares. For a quick lamp swap, the 50PCS kit is enough. If you go through a lot of one size, the bulk 100-pack bags come in single colours. When you wire trailer lights, seal plug backs and lamp entries with adhesive-lined dual wall heat shrink. If you are also wiring a caravan, read our guide to 12V caravan wiring repairs. Free express shipping Australia wide, with orders placed by 1pm dispatched the same day.