4 Pin Wiring Harness | etrailer Trailer wiring adapter changes the 4 pole flat on your vehicle into a 5 pole flat. Features: Length of harness: 20" Length of blue wire: 60" The blue wire gets spliced into the reverse light wire on the vehicle which is easily done with the included quick splice wire. Wiring Adapters. Single-Function Adapter. 4 Flat. 5 Flat. Tow Ready. 18 Inch Long. Value GM Accessories 12496599 7-Pin Round to 4-Pin Flat Trailer For trailers that require more than the standard 4-pin connection, use this GM Accessories Trailer Wiring Harness Adapter. This wiring harness adapter allows you to convert from a 7-pin round to a 4-pin flat connector. This GM accessory can help to personalize, customize, or optimize the capability of your vehicle for your lifestyle and use.