Insert the Wires: Loosen the screws on the terminal block using a screwdriver. Avoid overtightening, as this may damage the wire or the. The wiring diagram for a 12 pin trailer plug typically includes information such as the pin number, wire color, and function. This diagram acts as a guide, helping you to identify and connect the correct wires. It's important to carefully follow the diagram to ensure that all connections are made. Whether you're a beginner or an experienced trailer owner, this ultimate guide will provide you with step-by-step instructions on how to wire a 12 pin trailer plug.