Trailer Wiring Diagram – Lights, Brakes, Routing, Wires Typical Trailer Wiring Diagram and Schematic. These 2 wire diagrams fit the needs of most trailers. The image above shows a single axle trailer, and the next image shows wiring for Tandem Axles. Only the (blue) brake and (white) ground wires are different. Expand the same for additional axles. Use only the needed wires, and ignore the others.