2004 GMC Sierra Trailer Wiring | etrailer Lowest Price Trailer Wiring Guarantee. Installation instructions and lifetime expert support on all purchases of 2004 GMC Sierra Trailer Wiring. Order online at etrailer.com or call 800-298-8924. 2004 GMC Sierra 2500 HD Trailer Wiring Harness | Advance We currently carry 5 Trailer Wiring Harness products to choose from for your 2004 GMC Sierra 2500 HD, and our inventory prices range from as little as $14 up to $44. On top of low prices, Advance Auto Parts offers 1 different trusted brands of Trailer Wiring Harness products for the 2004 GMC Sierra 2500 HD.