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 1500 Trailer Wire Harness and Connector Equip cars, trucks & SUVs with 2004 GMC Sierra 1500 Trailer Wire Harness and Connector from AutoZone. Get Yours Today! We have the best products at the right price.