Wiring Cloud — Library of Wiring Diagrams & Standard Electrical Schematics

Wiring a towing hitch with a 4-pin flat connector

Australia's love for the outdoors means countless utes hitch up to trailers every weekend. Whether towing a runabout from Coffs Harbour or a camper across the Nullarbor, the electrical hookup keeps your brake lights, indicators and tail lamps working. A 4-pin flat connector is the simplest solution for lightweight trailers, and the job takes only basic hand tools.

The 4-pin plug carries four circuits: left turn, right turn, tail lamps, and a common earth. It is the standard fitment on most small box trailers, jet-ski dollies and camper setups sold through Australian retailers like Repco, Supercheap Auto and Narva stockists. If your trailer has brakes or a reversing light, you will need a 7-pin round connector instead.

Trailer wiring carries legal weight in Australia. The Australian Design Rules require all lighting on a towed vehicle to operate in sync with the tow car, and roadside inspections in states like Victoria and Queensland can issue defect notices for faulty trailer wiring. Work safely: disconnect the battery before cutting into any loom, and protect every joint from corrosion in our salt-laden coastal air.

This guide walks through the whole job — from choosing parts at your local auto shop to the final road test down a quiet street in Perth. You will learn the colour code, where to pick up a reliable body earth, how to route the loom, and what to do when a warning light flickers on the dash.

How the 4-pin flat connector works

Each pin has a fixed job, and the colour code is the same on virtually every trailer sold here. White is earth, brown feeds tail and number-plate lamps, yellow drives the left turn and brake lamp, and green drives the right. Brown stays on with the headlights, while yellow and green only receive power when you indicate or brake. For a primer on low-voltage wiring principles, the phone-wiring basics reference covers similar colour-coding conventions used in household cabling.

Because the brown circuit carries both tail and number-plate lamps, a single brown feed lights everything from the rear markers to the side reflectors without extra wires. This simple arrangement is why the 4-pin flat has remained the default for decades, and why you can swap a plug from one trailer to another without rewiring.

Tools and materials you will need

A typical job needs a pre-made 4-pin harness with about four metres of cable, a matching plug and socket, a self-tapping screw for the body earth, heat-shrink tubing, and automotive-grade loom tape. You will also need wire strippers, side cutters, a multimeter, a soldering iron or crimp connectors, and a socket set.

Most Australian auto shops stock pre-terminated harnesses from Narva, Projecta and Hella. If you would rather build your own, buy a four-core trailer cable plus a separate 4-pin plug and socket.

Items to put in the kit

Getting the tow vehicle ready

Find the tail-light wiring inside the boot. Most modern vehicles — from a HiLux in Darwin to a Ranger in Sydney — use multi-pin connectors at the rear lamp, and wire colours usually follow the convention: green for right turn, yellow for left turn, brown for tail, and black for earth. A multimeter confirms which is which before you cut anything.

Tap into the right circuits using solder and heat-shrink for a long-lasting repair, or Scotch-lock connectors for a quicker job. Run the earth wire to a clean, bare-metal point on the chassis, sand off any paint, and use star-washers under the earth screw. Coastal drivers should pay extra attention here, because a poor earth is the number-one cause of flickering trailer lights.

Route the harness along the chassis rail with P-clips every 40 cm, away from the exhaust and sharp edges. Drop the cable through a grommet in the boot floor, then bring it out near the tow bar. Leave a small service loop near the plug so you can unplug the trailer without straining the wires.

Wiring through to the trailer plug

On the trailer side, the job is the mirror image of the vehicle. Strip back the cable, identify each core by colour, and connect it to the matching pin on the 4-pin plug. A small screwdriver is usually enough to seat the wires in the screw terminals. Give each wire a gentle tug to make sure nothing will come loose on a corrugated dirt road.

Solder every joint where possible, then cover it with dual-wall heat-shrink tubing. If you must use crimp connectors, choose marine-grade heat-shrink types from a chandlery rather than cheap vinyl ones from a discount bin. Wrap the harness along the trailer drawbar with loom tape or split loom so stones, salt spray and mud cannot wear through the insulation.

Once the plug is wired, plug it into the tow vehicle and walk through every function with a helper watching the back of the trailer. Indicators left and right, brake lights, tail lights, and number-plate illumination should all operate together. If you are also fitting LED trailer lights, the boat navigation lights guide explains how low-current LED lamps interact with standard looms and why a dedicated load resistor can be needed.

Habits that make the job last

Testing the circuit and Australian compliance notes

Before a long tow, take the rig for a short drive around the block — somewhere flat like the streets in Melbourne or along the riverfront in Perth. Watch for any flicker from the trailer lamps as you indicate, brake, or drive over speed humps. A single flick points to a loose earth, while a constant dim glow suggests a voltage drop.

Australian regulations require all trailer lights to operate correctly and no wiring to chafe against the towbar. In New South Wales and Victoria a failed light can be grounds for a defect notice or a fail at rego time. Keep a spare 4-pin plug in the ute, because salt air tends to corrode plugs faster than the rest of the wiring.

Grab a pre-made harness from your local auto parts store this weekend, set aside a couple of hours on the driveway, and run through a careful test loop. With the right parts and a methodical approach, your 4-pin towing setup will be ready for the boat ramp, the campground, or the long run to the tip.