GMC Terrain Trailer Wiring | etrailer Trailer Wiring Harness Installation - 2015 GMC Terrain. Speaker 1: Today on our 2015 GMC Terrain, we're gonna be taking a look at showing you how to install the Curt vehicle wiring harness for factory tow package with a 4-pole flat trailer connector. 2019 GMC Terrain Hitch Wiring | Harnesses, Adapters GMC Terrain 2019, 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..