To legally tow a trailer, vehicles must be wired for trailer lights. To do so, you will need to check the connector and wiring from the vehicle to the trailer. This process addresses the proper wiring instructions for a vehicle using a four-way connector.
Locate the vehicle's tail-light wiring harness by removing the tail-light assembly on each side with a Phillips screwdriver or socket set. For trucks, remove two bolts on the side of the truck bed and pull out the tail-light assembly. For cars, the assembly is likely located inside the boot. Make sure the wiring harness will reach the trailer's connector receptacle. If it does not, you will need additional wiring.
Connect the clip end of the test light to the chassis on the rear of your vehicle as a ground. Turn on the vehicle's parking lights to test the wires.
Touch the test light to the wires in the wire harness at the back of the vehicle. This will help you find the wire that will operate the tail lights. Once the test light has a steady light, connect the brown wire to the wire that generated a constant light.
Turn off the parking lights. Crank the vehicle's engine, and turn on the right turn signal.
Touch the test light to the wires in the wire harness until you find the wire that operates the right turn signal. Once the test light has a flashing light, connect the yellow wire to the wire that generated the flashing light.
Turn off the ignition, wait a couple of seconds and crank the engine again. Repeat Steps 4-5 for the left signal.
Connect the remaining white wire to your ground wire.