A 4-way flat trailer wiring system consists of four wires that connect the trailer's lights to the tow vehicle's electrical system. These wires are typically color-coded for easy identification: white for the ground wire, brown for the running lights, yellow for the left turn/brake lights, and green for the right turn/brake lights.. The easy way to install a 4-way trailer wiring harness on your 1991 Ford F-150 is with the part # C55316 which will plug in line with your taillight wiring harness. Otherwise you'd have to use a circuit tester like part # PTW2993 to test the wires of your taillight harness and wire them accordingly to the 4-way. The 4-way wiring colors are:
2. A signal is going in and coming out on the appropriate wires, then there is a problem with the trailer wiring. Solve the problem by inspecting the wiring on the trailer to make sure all of the connections are correct and ground wires are connected properly. Most likely, the ground wire on the trailer is not secured properly.. The 4 flat trailer wiring diagram consists of four wires, each serving a specific function. These wires are color-coded for easy identification. The white wire is the ground wire and is responsible for providing a path for electrical current to return to the vehicle's battery. The green wire is the right turn signal and brake light wire.