How to Wire a Trailer: NZ 4, 5, 7 & 12-Pin Diagrams & Steps

Blown bulbs, flaky earths and mismatched plugs are common when trailer lights misbehave. Besides being unsafe, they can earn you a failed WOF or a fine. If you’re staring at a tangle of wires and wondering why indicators glow with the brakes, you’re not alone.

This guide shows you, step by step, how to wire a trailer the NZ way, with 4, 5, 7 and 12-pin flat diagrams, correct colour codes, and checks from planning to testing for safe, legal, reliable lights.

We’ll start with safety and legal checks, choose the right plug, decode NZ colours and pin numbers, gather parts and tools, route and connect the loom, add optional circuits (brakes, reverse, 12V aux), then weatherproof, test and troubleshoot.

Step 1. Check safety, legal requirements and plan your setup

Before you wire a trailer, stabilise the job and confirm legal basics. Hitch the trailer to your tow vehicle for stability, and list the circuits needed for WOF.

• ADR-compliant lights required in NZ.
• Minimum: two red tail lights, reflectors, indicators, number plate light.
• If vehicle lights aren’t visible: two brake lights. Over 2 m wide: marker/forward-facing lights.

Step 2. Choose your trailer plug type (NZ 4, 5, 7 & 12-pin)

When you wire a trailer, choose the plug to match your tow vehicle and the circuits you’ll run. In NZ, flat plugs are common and the 7‑pin flat is the standard. Need auxiliary circuits? Go 12‑pin. Only basic lights? 4‑ or 5‑pin may suit.

• 7‑pin flat — NZ standard; full road lights; reverse and electric brake pins available.
• 12‑pin flat — 7‑pin plus five high‑current auxiliaries.
• 4/5‑pin flat — basic lights only; use only if both ends match.

Step 3. Understand NZ wire colours and pin numbering

Getting colour and pin numbers right is the difference between crisp indicators and chaos. NZ trailers follow a consistent scheme; match the stamped numbers in your plug to the functions and colours below, and you’ll avoid mirrored wiring and mystery cross‑feeds when you wire a trailer.

NZ 7‑pin flat (standard): 1 Left indicator—Yellow; 2 Reverse—Black; 3 Earth—White (or Black); 4 Right indicator—Green; 5 Service/electric brakes—Blue; 6 Brake lights—Red; 7 Tail/number plate—Brown. On many basic trailers, pins 2 and 5 are left unused.

• • 4‑pin flat: Yellow (LH), Green (RH), Brown (Tail), White (Earth) — stop/turn combined.
• • 5‑pin flat: 4‑pin plus Reverse (Black) is common in NZ/AU.

Step 4. Gather parts and tools (plugs, harness, lights, connectors)

Gather everything first: a plug to match your tow vehicle (4/5/7/12‑pin), an ADR‑compliant light kit with number plate light, and a 6/7‑core harness. For clean, durable joins, have crimp connectors + heat‑shrink, cable ties and electrical tape, plus wire strippers, a terminal crimper and a heat gun.

Step 5. Sketch your wiring and measure cable runs

Sketch the trailer showing tow plug, left/right lamps and number‑plate light. Note the NZ colour and pin for each function (from Step 3). Choose a clean, central earth. Measure cable runs along one chassis rail, mark junctions, and leave slack for bends, drawbar movement and a small service loop.

Step 6. Remove old lights and wiring harness

Unplug from the vehicle and remove the old loom methodically. Photograph each termination, label wires, then cut ties/clips and pull the harness from the drawbar. Remove lamps and the earth screw, saving grommets. Clean earth points to bare metal, check for chafed sections, repair/conduit where needed, and smooth sharp edges before the new kit goes in.

Step 7. Mount new lights, reflectors and number plate light

Mount ADR‑compliant tail lamps on the outer extremities, level and square. The lamp with the clear window is for the number plate—orient the clear section toward and down onto the plate. Use supplied bolts or screws, drill/de‑burr and paint holes, and grommet cable entries. Fit red rear reflectors near each tail lamp before you wire a trailer further.

Step 8. Route the new harness and secure the earth

Route the 6/7‑core harness along one chassis rail from the plug to the tail, inside the rail where you can. Keep clear of heat and moving parts, grommet any pass‑throughs, and branch neatly to each lamp. Keep joins high and sheltered, and leave small service loops at the drawbar and behind each light.

• Secure every 300 mm with UV ties or P‑clips.
• Earth: tie lamp earths to the white (or black) and bolt a ring terminal to clean, bare metal—not the coupling.

Step 9. Wire the plug: 4-pin flat (NZ/AU)

A 4‑pin flat handles the basics, using combined stop/indicator circuits. Open the plug, feed the loom through the strain relief, strip about 10 mm, and terminate by colour. The white wire is the common earth for all lamps; keep strands neat and fully under each clamp.

• Yellow — Left indicator/stop (LH)
• Green — Right indicator/stop (RH)
• Brown — Tail/number plate
• White — Earth to chassis/common

Tighten terminals, clamp the cable, refit the cover, and tug‑test.

Step 10. Wire the plug: 5-pin flat (NZ/AU)

A 5‑pin flat adds a dedicated reverse circuit (Black) to the basic 4‑pin set. When you wire a trailer with a 5‑pin, match colour to function as below, using White for the common earth. If an old loom used Black as earth, move the earth to White. Strip 8–10 mm, seat conductors fully, then clamp the strain relief, refit the cover and tug‑test.

• Yellow — Left indicator/stop (LH)
• Green — Right indicator/stop (RH)
• Brown — Tail/number plate
• White — Earth
• Black — Reverse

Step 11. Wire the plug: 7-pin flat (NZ standard)

NZ’s 7‑pin flat is the standard when you wire a trailer for full road lights. Match the stamped pin numbers inside the plug to these colours. Pins 2 (Reverse) and 5 (Brakes) are often unused on basic trailers—cap them.

• Pin 1 — Left indicator — Yellow
• Pin 2 — Reverse — Black
• Pin 3 — Earth — White or Black
• Pin 4 — Right indicator — Green
• Pin 5 — Service brakes — Blue
• Pin 6 — Brake lights — Red
• Pin 7 — Tail/number plate — Brown

Tighten, clamp, refit cover, tug‑test.

Step 12. Wire the plug: 12-pin flat (top and bottom rows)

A 12‑pin flat combines the standard NZ 7‑pin lighting row with five auxiliary pins. When you wire a trailer, identify the row stamped 1–7 and connect it exactly as in Step 11 (same pin numbers, colours and functions). Use the stamped numbers rather than assuming by position. The other row provides extra feeds; Trailparts notes these pins are rated up to 35A each at 12V, so choose suitable cable, clamp the strain‑relief, and only connect auxiliaries your tow vehicle actually supports.

• Label both ends to match the vehicle’s socket diagram; leave unused auxiliary pins empty and insulated, then refit the cover and tug‑test.

Step 13. Add optional circuits: electric brakes, reverse and 12V aux

When you wire a trailer, you can add electric brakes, reverse and 12V aux—only if your tow vehicle feeds those pins. As a guide: 7‑pin poles ≈15A at 12V; 12‑pin top row ≈35A per pole. Fuse correctly, size cables for the load, and keep heavy‑load returns separate from lamp earths.

• Electric brakes: Pin 5 Blue (7‑pin); connect to the tow vehicle’s blue brake feed; return to Pin 3 earth.
• Reverse lights: Pin 2 Black (7‑pin); wire to reverse lamps; confirm the vehicle supplies this feed.
• 12V auxiliary: Use the 12‑pin top row per the vehicle’s socket diagram; fuse at source and run a dedicated return.

Step 14. Make weatherproof connections (crimp, solder, heat-shrink)

When you wire a trailer, moisture is the enemy—seal every join properly. Slide heat‑shrink over each wire before connecting, strip 8–10 mm, then crimp with quality crimp connectors using a terminal crimper. For extra security on static joins, add a light solder fillet. Centre the heat‑shrink over the crimp and shrink with a heat gun, then wrap with electrical tape and secure the loom.

Step 15. Test with a multimeter and your tow vehicle

Test with a multimeter first, then on the car. Keep the trailer hitched for a solid earth while you wire and test. This confirms your wiring plan works under load and catches issues before you seal everything up.

• Continuity and power: Pin 3 to chassis; each lamp feed to its pin; activate each function and verify power on the right pin.
• Function and wiggle: Tail, LH/RH indicators, brake, reverse (if fitted), number plate—all bright; gently flex loom and plug to expose intermittent faults.

Step 16. Troubleshoot common trailer light faults

Still got gremlins? Most trailer light faults are earths, broken wires at stress points, or pin‑mapping mistakes. Keep the trailer hitched so Pin 3 earth is solid, and use a multimeter as you activate each function and gently flex the loom and plug.

• Dim or backfeeding: fix the earth. Clean to bare metal.
• One side dead: check Yellow (Pin 1)/Green (Pin 4) and joins.
• No tails: Brown (Pin 7) open or check the tow vehicle fuse.
• No reverse/electric brakes: vehicle may not supply Pins 2/5; test and cap if unused.

Step 17. Maintenance and pre-trip checks

Even after you wire a trailer, reliability comes from pre‑trip checks: inspect plug pins and the socket for corrosion, tug‑test the strain relief, and look for chafe or loose ties along the loom. Confirm earth continuity, then test tail, brake, indicators and reverse. Clean/grease contacts and carry spare fuses.

Conclusion

That’s your trailer wired the NZ way: safe, legal and reliable. You’ve planned the circuits, picked the right plug, matched colours and pins, routed and sealed the loom, then tested and fixed any gremlins. Keep the pin map handy and run quick checks before each trip; it’ll save WOF hassles and roadside stress. If you need ADR‑compliant lights, 4/5/7/12‑pin plugs, looms, crimp and heat‑shrink kits, or advice, the team at Action Outdoors can help. Plug in, indicate, brake, reverse—everything bright and predictable.