This controller has a socket at one end for connecting to the trailer's 13-pin plug and at the other end for connecting to the car's 13-pin plug. It includes a 7 to 13-pin adapter. The 9V battery quickly indicates if there are any wiring issues with both the car and the trailer. It can be used to check the lights on caravans and bike racks. It features two outputs, one for the car output and one for the trailer input.