How to Wire a Four Prong Flat Trailer Connector

Light-duty cargo trailers use a four-prong electrical connector.

Most light-duty trailers are connected to a tow vehicle's electrical system by way of a flat, four-pronged wiring harness and connector. The tow vehicle uses this connection to power the running lights, brake lights and turn signals on the trailer. Without these lights, towing a trailer is dangerous and, in most states, illegal. Adding a four-wire harness to a vehicle is not difficult and can be done in about an hour with basic hand tools.

Things You'll Need

• Open-end or adjustable wrench
• Screwdrivers
• Trim removal tool
• Four-prong trailer wiring harness
• Wire stripper
• Ring connector
• Crimper
• Self-tapping screw
• Wire tap splices

Instructions

1. Disconnect the tow vehicle's negative battery cable with an open-end or adjustable wrench. Remove access or body panels behind the vehicle's taillights with a screwdriver and trim removal tool.

2. Locate the wires leading to the taillight assemblies. Strip 1/2-inch of insulation off the white wire, slide a ring connector onto the bare end and crimp it securely. Attach the ring connector to a grounded metal part of the vehicle's chassis using an existing bolt or self-tapping screw.

3. Locate the wire that powers the vehicle's left turn signal and splice the connector's yellow wire to it with a tap splice. Connect the green wire to the vehicle's right turn signal wire with a tap splice. Connect the brown wire to the wire that powers the vehicle's running lights with a tap splice.

4. Reconnect the vehicle's negative battery cable and tighten it securely. Connect the trailer plug to the connector in the tow vehicle and test the lights for proper operation. Recheck all connections if the lights fail to operate correctly.

5. Tuck the wiring harness behind the removed panels, leaving the connector in an easily accessible area of the trunk or luggage compartment. Reinstall the access or body panels that were removed to access the vehicle's wiring. Place the connector in a safe, dry location in the vehicle when not in use.

Tips & Warnings

• To prevent corrosion or damage to the four-pronged connector, leave it inside of the tow vehicle when not in use. Run the wires out through the tailgate or trunk lid and close the lid gently on the wires when connecting the vehicle to a trailer. The weather stripping on the trunk lid or tailgate will protect the wires from damage.

• If your vehicle uses separate bulbs for brakes and taillights, a converter will need to be installed before connecting the wiring harness.

References

• E Trailer: Trailer Wiring Diagrams