Trailer Brake Electric Reverse Lock Out Solenoid. Simply attach the switch/solenoid to your trailer brake actuator and wire to the ground and reverse light of the towing vehicle. Allows your trailer to freewheel when backing up, so you won't have to get out to flip switches. Can also be used on drum brakes. This solenoid has two wires, one for power and one for ground so it does not have the poor grounding issues of the single wire solenoids.