2012 GMC Sierra Trailer Wiring | etrailer Call 800-298-8924 to get Expert Service ordering a Trailer Wiring for your 2012 GMC Sierra. Complete installation instructions and lifetime technical support on all Trailer Wiring purchases. Lowest price guarantee on accessories for your GMC Sierra and the fastest shipping available. 2012 GMC Trailer Wiring | etrailer If your 2012 GMC Acadia was equipped with the factory installed 7-way trailer connector at the rear of the vehicle, then installing a brake controller will be a plug-and-play process. You'll need the # 3026-P brake controller wiring harness, as you mentioned.