Trailer Wiring Diagram: How to Wire 4, 5, 7 & 13-Pin Plugs

A 4-pin trailer plug feeds the basics: tail, stop and indicator lamps. Add a fifth wire and you can run reverse lights or a hydraulic brake-lockout for boat launches. Seven pins unlock electric service brakes, auxiliaries and fog lamps, while the 13-pin Euro connector gives each circuit its own earth plus separate 12-volt feeds for caravan fridges and batteries. Choose the wrong plug— or wire the right one incorrectly— and WoF inspectors, blown fuses or late-night roadside darkness soon let you know.

Wiring a trailer isn’t hard once you understand the colour codes, the pin numbers and a handful of best-practice tips for crimping, sealing and testing. This step-by-step guide walks you through legal requirements in New Zealand, the tools you’ll need, and clear diagrams for 4, 5, 7 and 13-pin plugs. By the end you’ll have road-ready lights, brakes that cut in when they should, and the confidence to tow safely anywhere from Cape Reinga to Bluff.

Step 1 – Check Legal Requirements and Choose the Right Plug

Before you strip a single wire, make sure the connector you’re planning to fit will satisfy New Zealand’s Land Transport Rule: Lighting 2004 and keep a WoF inspector happy. The rule dictates which lamps must function on trailers of different sizes and weight ratings, so the plug you pick must be able to power every mandatory circuit—now and after that “one-day” upgrade to electric brakes.

NZ lighting rules every trailer owner must meet

For trailers under 3.5 t and narrower than 2.1 m you legally need:

• tail/park, stop and both indicators
• a white front marker if over 2.1 m long
• number-plate lamp and at least one red reflector

Wider or heavier rigs also require reverse lamps, side markers and an independent service-brake circuit. Inspectors check for:

1. correct lamp colour and intensity
2. simultaneous operation with tow vehicle
3. secure cabling with no exposed copper

Matching plug types to trailer tasks

• 4-pin: garden carts, quad bike trailers—basic lighting only
• 5-pin: boat trailers needing reverse-lockout or dedicated reverse light
• 7-pin: braked horse floats, builders’ trailers, small caravans
• 13-pin: European caravans/motorhomes needing fridge and battery feeds

Choosing a larger plug than you strictly need today gives headroom for future accessories.

Flat vs round, plastic vs metal housings

Flat plugs (AS/NZS 4735) dominate in NZ; they sit higher, resist stone strikes and parts are everywhere. Round ISO plugs mate with many imports but can drag on steep driveways.
Plastic housings shrug off salt but their screws strip easily; metal shells are tougher and shield against electrical noise yet cost more and need dielectric grease to avoid corrosion.

Step 2 – Gather All Tools, Parts and Safety Gear

Having the right kit on the bench prevents half-finished jobs and sketchy joins. Before you open the plug body, lay out every cable, connector and safety item you’ll need so you can work methodically and keep copper clean.

Wiring and hardware checklist

• 5-, 7- or 13-core automotive cable (1 mm²–2.5 mm², tinned if near salt water)
• Correct plug or socket bodies plus dust caps
• Junction box or heat-shrink sleeves for chassis joins
• Split conduit, cable clips and glands for abrasion protection
• Dielectric grease, suitable blade fuses and labels for each circuit

Essential tools

• Quality wire strippers and ratchet crimper
• Mini flat-blade screwdriver for plug terminals
• Multimeter or 12 V test lamp
• Heat gun and solder (optional for sealed joints)
• Side cutters, cable ties and a sharp craft knife

Safe work practices

• Disconnect the tow-vehicle battery and chock wheels
• Wear eye protection and insulated gloves when soldering
• Work on a dry, well-lit bench—avoid gravel driveways and wet lawns
• Keep a fire extinguisher within reach when using a heat gun or torch

Step 3 – Understand NZ Colour Codes and Pin Numbers

Colour discipline is what keeps every trailer wiring diagram honest. A pin that sends power to a brake light on one rig must never feed a fog lamp on another, or chaos (and blown fuses) follows.

Standard 7-core chart (AS/NZS)

Below is the go-to reference used by NZ WoF inspectors. The flat plug keeps the same colours as the small-round style, but the pin numbers swap sides—so always wire by colour first, number second.

4- and 5-core variations

When you drop pins, you drop colours: blue (brake) and black (reverse) disappear first, leaving yellow, green, brown, red and the all-important white earth. Watch out for older imports using green for tail and brown for right indicator—double-check with a test lamp.

European 13-pin specifics

The Euro plug doubles up on earths (pins 3 & 13) and adds:

• Pin 9 = +12 V permanent (20 A fuse)
• Pin 10 = +12 V ignition/fridge (15 A)
• Pin 2 now runs the rear fog lamp.
  Wire sizes jump to 2.5 mm² for the fridge feed to prevent voltage sag.

Reading the moulded diagram

The tiny graphic inside each housing is shown “from the rear of the plug”, i.e., the solder/terminal side. Imagine holding the plug at arm’s length and reading it like a clock: pin 1 at 9 o’clock on a flat, 1 o’clock on a round. Use that mental picture and you’ll never cross wires again.

Step 4 – Wire a 4-Pin Flat Plug (Basic Lighting)

Four-core cable means one earth and three live feeds—ideal for garden trailers and small utes.

Strip, crimp and secure conductors

Strip 6 mm of insulation, twist strands, crimp open-barrel pins, then tighten screw clamps to about 0.5 Nm and refit the cord grip.

Connect tail/park, stop and indicators

Brown tail, red stop, yellow left, green right. Tie all lamp negatives into the single white earth inside a sealed junction box.

Quick bench test

Bench-test with a 12 V battery: each lamp should glow cleanly and nothing else flicker. Dim lights usually mean a dodgy earth.

Step 5 – Wire a 5-Pin Flat Plug (Adding Reverse or Brake Lock-Out)

The 5-pin set-up is the same as a 4-pin trailer wiring diagram, with one extra live feed to handle either a reverse lamp or a hydraulic brake-solenoid. That little bonus wire means safer late-night backing and no salty dunking of drum brakes at the boat ramp.

Identifying the extra core

• NZ standard uses black for reverse.
• Some marine looms flip it: blue for the brake-lockout solenoid.
  Match the loom colour to the device, never the paint job.

Recommended conductor size and fuse

Run at least 1.5 mm² cable for the extra circuit and protect it with a 10 A blade fuse close to the tow-vehicle battery.

Real-world example: boat trailer

1. Black wire to actuator solenoid.
2. White earth bonded to galvanised draw-bar.
3. Heat-shrink, then route inside split conduit along the A-frame to keep water and stones out.

Step 6 – Wire a 7-Pin Plug (Flat & Round) for Braked Trailers

Seven cores give you everything a WoF inspector could ask for: indicators, tail, stop, reverse, plus a dedicated service-brake feed. Whether you choose the common NZ flat style or the ISO round socket fitted to many imports, the wiring logic is identical—only the clock-face layout shifts. Use the tables below, wire by number and colour, and you won’t trip over yourself swapping trailers at the ramp.

Pin-out reference

7-pin flat (AS/NZS 4735):

7-pin round (small & large, ISO 1724):

On un-braked utility trailers the blue (round) or black (flat) wire is often parked inside the plug—tape it off so it can’t short against the shell.

Adding electric drum brakes

Run the blue service-brake wire (flat Pin 7, round Pin 2) in 2.5 mm² twin-sheath back to the axle junction box. Protect the circuit with a 20 A self-resetting breaker near the in-cab controller and earth the brake magnets to the chassis, not the lamp earth, to avoid voltage drop.

Using auxiliary Pin 2

If you don’t need electric brakes, that spare pin is a handy bonus. Common re-purposes include:

• Rear fog lamp on imported trailers
• Interior float or toolbox light
• 12 V accessory feed via a relay (max 8 A)

Label the socket lid or keep a note in the glovebox so the next driver knows what to expect.

Step 7 – Wire a 13-Pin Euro Plug for Caravans & Motorhomes

European caravans landing on our shores already wear the 13-pin connector, and many Kiwi builders of larger horse floats are switching too. The single, twist-lock housing is dust- and splash-proof, carries more current than twin 7-pins, and satisfies ADR/UNECE rules without adapters that love to fall out on corrugations.

Why 13-pin is catching on in NZ

• One watertight plug instead of two 7-pin sockets
• 30 A rated contacts for fridge and battery charging
• Extra earth returns reduce voltage drop to LEDs and ABS modules

Dedicated circuits you gain

Step-by-step wiring procedure

1. Unscrew the rear shroud and slip the cable gland over the loom.
2. Strip 45 mm of outer sheath, then 6 mm from each core.
3. Crimp ferrules; land earths (pins 3 & 13) first to anchor the bundle.
4. Working clockwise from pin 1, insert each conductor, ending with pins 9–11 so the heavier wires sit at the top.
5. Tug-test every wire, refit the strain-relief clamp and smear dielectric grease round the O-ring before snapping the shroud home.

Converting from 7- to 13-pin

A cheap adapter lead will light indicators, tail and brakes but won’t power the fridge or house battery, and you’ll still have two earths sharing one skinny wire. For reliable charging and fog-lamp legality, run a fresh 13-core cable and re-terminate the trailer junction box—your future self will thank you.

Step 8 – Test, Troubleshoot and Weather-Seal

Before you hitch up, spend ten minutes proving that every circuit shown on the trailer wiring diagram actually delivers clean volts and amps. Systematic testing now beats roadside panics later.

Test sequence

1. Continuity: unplug the trailer, probe each pin to its lamp with a multimeter; resistance should read under 1 Ω.
2. Voltage under load: energise each circuit, check for at least 12.2 V at the lamp socket while the bulbs glow.
3. LED polarity: if an LED won’t light, reverse the probe leads—diodes only flow one way.
4. Brake draw: with brakes applied, clamp-meter the blue wire; typical electric drums pull 3–4 A per axle.

Common faults and quick fixes

• Dim or flashing indicators → repaint chassis earth or add a dedicated white return wire.
• One lamp cuts in and out → broken conductor where the draw-bar pivots; splice in fresh cable.
• Fuse pops instantly → chafed insulation; sleeve with split conduit and re-route away from steel edges.

Lock moisture out for good

• Pack dielectric grease around each pin before closing the plug.
• Slide a double-wall heat-shrink boot over the tail and shrink until adhesive oozes.
• Wrap the exposed loom in spiral conduit along the A-frame and retorque the cable clamp to hand-tight.
  A dry, strain-relieved cable is a long-lived cable.

Step 9 – Ongoing Maintenance and Smart Upgrades

A tidy plug today can turn feral inside a single muddy season, so lock in a maintenance habit and think ahead about upgrades that make towing easier and safer.

Inspection timetable

• Quick glance every trip: cracked lenses, loose plug, chafed loom
• Six-monthly: remove plug cover, check screw torque, regrease pins, test all lights
• Annual: pull split conduit, look for green corrosion, replace any cable with brittle insulation

LED light conversion

Swapping old bulbs for sealed LEDs slashes current draw and shrugs off vibration. Because LEDs are polarity-sensitive, confirm the trailer wiring diagram follows standard earth placements and, if indicators hyper-flash, fit a low-load flasher relay or a ballast resistor.

Handy extras

• Break-away switch tied to the tow-ball safety chain
• 13-pin to Anderson lead for DC-DC charging while you drive
• Spare cores repurposed for a wireless reverse-camera feed or tyre-pressure repeater

Upgrades like these add only minutes to a rewiring job but years of convenience down the track.

Quick-Reference Diagrams and Fuse Chart

Screenshot this cheat-sheet and keep it in the glove box.

* Fuse sits near the tow-vehicle battery to protect the loom.
Colour key: Y = Yellow, G = Green, Br = Brown, W = White, Bk = Black, Bl = Blue, R = Red, Or = Orange, Gy = Grey.

Keep Your Trailer Lights Legal and Bright

Keeping your trailer wiring diagram handy is half the battle. Pick the plug that suits the load, stick to the NZ-standard colours, crimp and seal every joint, then run a full lighting test before each tow. Once a year crack the plug open, clean corrosion and re-torque the terminals; replace any cable that’s gone stiff or green. Follow that simple routine and WoF inspectors will nod you through, while other motorists actually see your signals.

Need quality parts? Grab flat or round plugs, automotive cable, weatherproof LEDs and dielectric grease straight from the shelves at Action Outdoors and spend your weekends towing, not troubleshooting.