Trailer Wiring Harness Installation - 2003 Chevrolet Jun 29, 2010How to Install a Wiring Harness for a Trailer on a 2003 Chevrolet Express Van Today on this 2003 Chevrolet Express Van we are going to install part number 31345 from Hopkins. This wiring harness plugs into the existing factory harness and the connection points are located inside the vehicle, below both tail lights.4.7/5(27)Category: Custom Fit Vehicle WiringBrand: Hopkins 2003 chevy express 3500 passenger van bought used with Sep 14, 20082003 chevy express 3500 passenger van bought used with 3500 miles. Instrument cluster lights not working. Repaired by - Answered by a verified Chevy Mechanic. What I need is an electrical wiring diagram for the 2003 chevy express van to figure out the wiring problem. It's not the switch like you answer suggested.