How To Do 7 Pin Trailer Plug Wiring: NZ Colour Code Guide

You’ve bought the trailer, loaded the gear and the boat ramp is calling—yet the indicators glow dim or not at all. Nine times out of ten the culprit is the humble 7-pin plug, mis-wired or corroded after a few salty summers. Faulty lights aren’t just annoying; they can attract an on-the-spot fine, void insurance after a smash and even sideline your warrant. Sorting the plug takes less than an afternoon, provided you follow the right colour code and give each joint the respect it deserves.

This guide lays everything out in plain Kiwi English. First you’ll get a “cheat-sheet” pin chart so you can wire the plug without scrolling. From there we cover the standards NZTA inspectors look for, the tools that make crimps bullet-proof, and a pin-by-pin walkthrough that works whether you’re rewiring a box trailer or fitting brakes to a tandem caravan. By the end you’ll have a tidy, weather-proof connection and the confidence to tow anywhere in Aotearoa.

Step 1: Confirm NZ Standards and Identify Your Plug Type

Before stripping a single wire, make sure you’re working to the rule-book that NZ Police and WOF inspectors use. The standard dictates which circuit sits on which pin, so guessing isn’t an option.

Know the rules: NZS 5431, ADR & NZTA requirements

New Zealand follows the Australian Design Rule 7-pin flat layout, referenced in NZS 5431 and endorsed by NZTA. You must provide indicators, stop, tail and number-plate lamps; electric service brakes become compulsory once the trailer’s gross trailer mass exceeds 2000 kg. Reverse lights aren’t mandated but are strongly advised for anything you’ll back at night.

Flat vs small-round 7-pin: similarities, differences, adapters

Both plug shapes carry the same seven circuits and colour code, but pin positions differ:

• Flat – wider, easy to clean, sits flush against bumper
• Small-round – compact, less snag-prone in mud
• Pros/cons equal out, so choose what matches the tow vehicle; carry a flat/round adapter when touring.

Check what’s already fitted to your tow vehicle

Flip the socket lid—most have “7 PIN FLAT” or “ROUND” moulded underneath. Count the holes: straight line = flat, circle = round. If the ute uses a factory 12-pin, the lower row is the 7-pin standard, so wire only those contacts.

Step 2: Gather the Right Tools, Components and Safety Gear

A tidy, long-lasting loom starts with quality materials and the right gear within arm’s reach. Lay everything out on a clean bench before you slice the sheath—nothing stalls a wiring job faster than hunting for a missing crimper mid-heat-shrink.

Complete shopping list

• 7-core automotive cable (tinned copper, PVC jacket, ADR compliant)
• 7-pin plug or socket — flat or small-round, nylon or cast alloy
• Assorted heat-shrink tubing (3:1 adhesive-lined)
• Dielectric grease and a cable gland or rubber boot
• Split conduit, UV-stable loom tape and cable ties
• Optional extras: inline fuse holder, electronic brake controller, LED trailer tester

Essential tools for a clean, durable job

• Wire strippers and sharp side cutters
• Ratchet crimpers with matching insulated terminals
• Small flat-blade screwdriver for plug screws
• Multimeter and 10 mm spanner for battery clamp
• Heat gun (or lighter in a pinch)
• Optional soldering iron and rosin-core solder

Safety first

Disconnect the vehicle’s negative battery terminal, don safety glasses and keep stray strands clear of steel edges. Route wiring clear of exhausts, fuel lines and moving suspension parts; secure every 300 mm with ties or P-clips to stop chafe before it starts.

Step 3: Decode the 7-Pin NZ Colour Code Before You Cut Anything

Before the side-cutters touch the sheath, commit the New Zealand colour code to memory. The plug may look symmetrical, but mixing up two wires will have the right-hand indicator flashing when you brake—guaranteed to annoy following traffic and fail a WOF on the spot. Keep this cheat-sheet on the bench and you’ll wire the 7 pin trailer plug once, not twice.

Pin-to-Colour-to-Function Reference Table

Why Colours Matter — And When They Don’t

Manufacturers follow the chart above, but imported trailers and DIY looms sometimes swap black and blue or delete the reverse wire altogether. Salt, UV and grime can also tint the insulation, so never rely on sight alone. Strip a tiny section and probe with a multimeter; voltage never lies, paint sometimes does.

Frequent Questions Answered

• Are all 7-pin trailer plugs wired the same? In New Zealand, yes—both flat and small-round use this pattern, even though the pin positions differ.
• What if my loom has a purple or orange wire? Those are non-standard feeds (often 12 V accessory) and should be connected only if your tow vehicle provides a matching socket.
• Can I leave the blue brake wire unused? Absolutely; cap it with heat-shrink so stray strands don’t short to the plug body.
  Keeping these nuances straight now saves hours of head-scratching later.

Step 4: Prepare Cables and Terminals for a Long-Lasting Connection

Good wiring isn’t only about getting the lights to blink now—it’s about having them still blink after thousands of corrugated-gravel kilometres and a few dunkings at the boat ramp. The secret is to treat the 7-core cable like any other moving part: give it the right length, protect it from rub points, and make every joint mechanically solid before it ever carries current.

Measure twice, cut once

Run the cable along the draw-bar with the trailer coupled and jack-knifed hard each way. Mark a route that leaves just enough slack for tight turns while keeping the loom 150 mm above the ground. Add 150–200 mm at the plug end for future re-terminations. Where the cable passes through steel, fit a rubber grommet or plastic bulkhead gland to stop the chassis sawing through the PVC sheath.

Strip, crimp or solder — achieving solid electrical contact

Strip 5–7 mm of insulation with a sharp stripper; avoid nicking strands. Twist the copper, insert into the correct ferrule or insulated splice, then use a ratchet crimper until the tool releases automatically—no half measures. Solder is optional: if you go that route, crimp first for mechanical strength, add solder, then slide adhesive-lined heat-shrink over the joint and shrink until adhesive beads out.

Weatherproofing and strain relief

Before closing the plug, smear dielectric grease around each terminal; it repels salt and stops galvanic corrosion. Tighten the built-in cable clamp so you can tug the sheath without feeling movement at the pins. Dress the loom in split conduit, tape the ends with UV-stable loom tape, and tie it to the chassis every 300 mm with nylon cable ties turned so their heads face away from tyre spray.

Step 5: Connect Each Wire to the Trailer Plug — Pin-by-Pin Walkthrough

Lay the plug body on the bench with the locating notch at the top (flat) or the alignment key at twelve-o’clock (small-round). The pins are always numbered in a clockwise spiral starting at the notch/key, so read the tiny figures moulded beside each screw before slotting a wire. Work methodically: strip, twist, tin or crimp, grease, then tighten. A small dental mirror or phone camera helps confirm no stray whiskers are touching neighbouring brass.

Pin 1 – Left-hand indicator (Yellow)

On a flat plug it’s the outer left pin; on a round it sits immediately clockwise from the key. Insert the yellow core, nip the screw until the conductor doesn’t rotate, then flash the hazard lights to verify the filament blinks at normal cadence.

Pin 2 – Reverse lamps (Black)

Many box trailers skip this circuit, but wiring it now saves grief when you upgrade to LED reversing floods later. With the vehicle in reverse (ignition on, engine off) you should read 12 V between this pin and the white earth.

Pin 3 – Earth/return (White) — the backbone of the system

Fit the largest wire first so the clamp seats squarely. Give it an extra tug test; 90 % of lighting faults trace back here. If the trailer frame is galvanised, add a second earth strap to a clean, bolt-through lug.

Pin 4 – Right-hand indicator (Green)

Mirror image of Pin 1. After tightening, run hazards again to confirm both sides blink together. Rapid flash indicates an open circuit or LED load mismatch.

Pin 5 – Service brakes (Blue)

Connect only if your trailer has electric or electric-over-hydraulic brakes and a cab controller. For currents above 15 A upgrade this single conductor to 5 mm² and route it through split conduit separate from the light loom.

Pin 6 – Stop lamps (Red)

Stops share a single feed; a weather-sealed junction box on the draw-bar keeps the splice dry. Press the brake pedal while a helper watches for equal brightness both sides.

Pin 7 – Tail & number-plate lamps (Brown)

Continuous-duty circuit, so keep the fuse small (5 A). If you’re adding extra marker lights, daisy-chain them here with solder-sealed butt joins, not Scotch-locks, and check they don’t dim when you indicate.

Step 6: Wire the Tow Vehicle Socket or Install an Adapter Harness

Before a bulb ever lights, power must leave the ute correctly. Whether you take the factory-loom shortcut or splice the wires yourself, the goal is the same—deliver the right voltage to the right pin without introducing future fault points.

Plug-and-play vs hard-wire: which to choose?

• Late-model utes and SUVs often hide a dedicated trailer plug behind the rear bumper; simply clip in a pre-terminated loom and you’re done.
• Older or imported vehicles rarely have that luxury, so you’ll need to hard-wire into the tail-light cluster. Match the vehicle wire colours to the NZ trailer code, not to US or European patterns shown online.

Accessing the loom safely

Remove the tail-light housing with a plastic trim tool, not a screwdriver, to avoid cracking lenses. Feed the 7-core cable through an existing rubber grommet; if you must drill, de-burr the hole and fit a watertight grommet immediately.

Circuit protection and polarity checks

Locate the factory trailer-light fuse—Ford Ranger and Hilux both hide it in the engine-bay box. Verify 12.6 V on each socket pin with a multimeter and ignition on. Record your readings; inconsistent voltages usually trace back to a blown fuse or corroded earth.

Step 7: Test, Troubleshoot and Stay Legal on NZ Roads

Before you declare the 7 pin trailer plug wiring job finished, prove it under load. A quick driveway test catches crossed wires and weak earths long before an NZTA inspector—or the car behind you—does.

Functional test sequence

Grab a mate and work through these checks in order:

1. Left indicator
2. Right indicator
3. Hazard lights (both sides together)
4. Tail and number-plate lamps
5. Brake lights (foot on pedal)
6. Reverse lamps
7. Electric brakes (apply manual override if fitted)

Diagnosing common issues

• Rapid indicator flash ➜ blown bulb or dodgy earth—clean Pin 3 and retest.
• All lights dim when brakes applied ➜ undersized earth; upgrade white wire.
• LEDs glowing faintly when off ➜ back-feed; fit 680 Ω load resistor per side.

Final compliance check

Walk around at night: every lens lit, plate readable, loom secured at least 150 mm off the deck and no exposed copper. Tick those boxes and you’re WOF-ready for any Kiwi road or ramp.

Keep Your Wiring Road-Ready

Trailer electrics aren’t a fit-and-forget job. Salt spray, UV and vibration will eventually find the weakest joint, so give the loom a quick once-over every time you grease the hubs. Check the plug face for bent pins, wipe off grit and top up the dielectric grease if it looks dry.

Once a season, run the full light test with a helper and jiggle the draw-bar while the lamps are on; flickers reveal hidden breaks long before they strand you at night. Look for chafe marks where the cable brushes the chassis, tighten loose cable ties and replace any that have gone brittle. If you launch in salt-water, rinse the plug and socket with fresh water, shake out the droplets and let them air-dry before re-connecting.

Need quality plugs, tinned 7-core cable or a pocket LED tester? Drop into the Auckland store or browse Action Outdoors for gear that lasts the distance.