5 Pin Trailer 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 Amazon: 5 flat trailer wiring harness Jili Online 5 Pin Trailer Flat Plug 960mm Wiring Harness Connection Kits for Boat Car RV American Style - Black 1, 960mm. $9 $ 9. 99. FREE Shipping. Only 16 left in stock - order soon.