Abstract: An adapter unit for interfacing the existing brake and turn signal light system of a vehicle with the light system of a trailer. The adapter unit includes a first relay having a single coil and a pair of contacts relatively movable to a closed circuit condition in response to energization of the coil and biased to an open circuit condition. A second relay has a single coil and a pair of contacts also relatively movable to a closed circuit condition in response to energization of its coil and biased to an open circuit condition. The coils are interconnected with the vehicle light system. The contacts are interconnected with the trailer light system and are also connected to the vehicle battery through a different circuit than the one including the turn signal flasher of the vehicle.