GMC Sierra Trailer Wiring | etrailer Trailer Wiring Harness Installation - 2013 GMC Sierra. Today on our 2013 GMC Sierra 3500 we're going to be showing you how to install the Bargman 5th wheel/goose neck 90 degree wiring harness with the 7 pole plug. This is a 9 foot long extension. It's part number is 50-97-410. We need to find a suitable mounting area for our wiring. gmc trailer wiring harness | eBay 2 product ratings - Trailer Hitch Wiring Tow Harness For Chevrolet Colorado & GMC Canyon Part#118301 $32 Trending at $36 Trending price is based on prices over last 90 days.