GMC Terrain Trailer Wiring | etrailer Trailer Wiring Harness Installation - 2012 GMC Terrain. Speaker 1: Today, on our 2012 GMC Terrain, we're gonna be taking a look at, and showing you how to install the Curt T-Connector Vehicle Wiring Harness for Factory Tow Package with a Four-Pole Flat Trailer Connector. 2018 GMC Terrain Hitch Wiring | Harnesses, Adapters wwwd››Trailer Hitches›Hitch Wiring›GMC›TerrainGMC Terrain 2018, Towing Wiring Harness by Tekonsha®. One-year warranty. 4-Way Flat. Wire up your tow vehicle and trailer with whatever it takes to get your trailer lights on, the electric brakes slowing down, and the trailer battery..