[PDF] Trailer Wiring Diagrams | etrailer - ActionOutboards 7-Way Round Trailer Connector (Flat Pin) 7-Way Round Vehicle Connector (Flat Pin) 7-Way Round Trailer Connector (Round Pin) 7-Way Round Vehicle Connector (Round Pin) 7-Way Connectors Aside from the three main lighting functions, additional pins for electric brakes, a 12 volt "hot" lead, and backup lights are available. There are two types of 7 Parts of a Trailer Hitch - Essential Towing Equipment - Towing 101 A trailer wiring harness is a length of wire at the back of a vehicle that allows its electrical system to connect to a trailer. Integrated with the vehicle's existing wiring, it provides a standard connector, such as a 4-way flat plug. A trailer wiring harness is one of the most vital towing parts but can often be overlooked.