12 Volt Relay Wiring Diagram | Free Wiring Diagram Assortment of 12 volt relay wiring diagram. A wiring diagram is a simplified traditional pictorial depiction of an electrical circuit. It reveals the components of the circuit as simplified shapes, and also the power and signal connections in between the tools. Wiring Tips: Using Relays | Offroaders The method I use for wiring the lights and other external accessories, for the most part, follows the diagram pictured above. As in the diagram a wire is run from a 12 volt power source to the switch in the cab and out to the relay placing a fuse at the source of the power.