How to Wire Lights on a Trailer | Wiring Diagrams Trailer Wiring Diagram. To connect the electric system of your trailer to the vehicle, you will be using special connector. Above we have describes the main types of trailer wiring diagrams. Below is the generic schematic of how the wiring goes. 4 pin trailer wiring diagram5/5(2) Trailer Wiring Diagram – Lights, Brakes, Routing, Wires The trailer wiring diagram shows this wire going to all the lights and brakes. And, it must connect with things (if included) that use the Aux Power and Back-up lights too. Some trailer builders just connect this wire to the frame, then connect the ground from all the other lights and accessories to the frame as well.