Four Pin Trailer Plug Schematic - Best Place to Find Four pin trailer plug schematic Require you to read those schematic diagrams that have tiny little wires labeled with type too small for even a lawyer to read. Got a 7 pin connector on the truck and an inline 4 pin connector on Or outings. It has a trailer hitch Trailer towing used to be simple and straightforward. 4 Pin 7 Pin Trailer Wiring Diagram Light Plug | House 4 Pin 7 Pin Trailer Wiring Diagram Light Plug . 4 pin trailer light wiring diagram : 4-Pin Connectors. So, do not run the wire from the bumper or the hitch of the car. The 4-pin standard flat style connector has four contacts with a male pin on the vehicle side for the ground.