This allows the TLC box to be hooked up to a boat trailer, should there ever be a need to run its lights independent of the TV. The harness consists of a vehicle-side 4-wire flat plug and a 12V extension cord, both wired into a trailer-side 7-way Bargman plug. All parts were from Wal-Mart. The 12V plug provides power when plugged into the boat's 12V outlet. It is important to wire the ground from the 4-wire harness together with the ground from the 12V cord and pin 1 of the bargman plug.