US Trailer Wiring Guide

Interactive 4-Pin, 5-Pin, 6-Pin & 7-Pin connector diagrams with top-down trailer view

▶

4-Pin Flat Connector

Click a wire or legend item to highlight the circuit on the trailer
Connector View
Light Type
Quick Test

Top-Down Trailer View

PLUG TAIL TAIL L TURN R TURN REV REV ELEC BRAKE 12V AUX GROUND FRONT REAR L MARKER R MARKER
Symptom Preview

    Select a symptom to walk through a step-by-step diagnostic procedure with multimeter instructions and probe placement.

    Calculate the recommended minimum wire size (AWG) to prevent dim lights and overheated wires based on your trailer's length and lighting setup.

    Select your tow vehicle to find out what factory harness it likely has, and what adapters you may need.

    The majority of trailer lighting problems are caused by poor wire splices. Compare the right way vs the wrong way to securely join wires together.

    ✅ The Right Way (Marine Grade Crimp & Shrink)

    Provides a watertight seal that resists vibration and corrosion.

    1. Strip exactly 1/4" to 3/8" of insulation from both wires without cutting the copper strands.
    2. Insert into an uninsulated or heat-shrink butt connector. Ensure all strands pass inside the metal barrel.
    3. Crimp using a ratcheting crimp tool. Do not crush the barrel flat; use a proper dimple die.
    4. Heat Shrink using a heat gun until the adhesive oozes out the ends, fully sealing the joint from moisture.

    ❌ The Wrong Way (Twist & Tape)

    Guaranteed to fail eventually underneath a trailer where moisture and vibration are constant.

    • Twisting wires only provides a weak mechanical bond. Over time, vibration will pull them apart.
    • Electrical tape inevitably unwinds or turns to slime when exposed to road grime.
    • Wire nuts are designed for solid copper house wiring, not stranded automotive wire bouncing down a highway.
    • Scotch Locks bite into the wire, but leave the copper exposed to air and salt, causing green corrosion within months.