atboley

I just experienced the same problem with no tail lights on a 2001 Silverado 3500 with a Heil dump body & this post was immensely helpful finding & resolving my issue.  I checked all the suspect deficiencies that atboley mentioned.  I eliminated the light switch since the front park, instrument & cab clearance lights worked.  I found & powered the tail light wire near the rear of the truck and all lights lit.  Given all lights were out, I figured it was the whole circuit (fuse or fuses), wire forward of the test point or a relay.  I checked all fuses and replaced the park lamp relay under the dash - still no lights.  I examined the entire wiring sheath that was visible along the frame & didn't see anything obvious.  Wires were still taped/cable tied inside the OEM wire loom & it showed no sign of damage.  Finally, I broke into the wire loom starting from the rear where I had been successful powering the tail lights & worked forward.  I checked several places in the wiring harness, starting from the rear and found a live tail light wire by attaching a test light & turning on the light switch.  That was about 3' from the rear.  This was the last place I expected to find the problem since the wire loom and wiring harness showed no sign of damage from that point to the rear of the truck.  I have yet to tear into the wire loom and find the exact spot where the tail light wire is interrupted because I was away from my shop & simply spliced a new wire into the tail light wire at the known good point & bypassed the defect.  It still is a mystery why the tail light wire inside a continuous, straight wire run with no connectors or bends or crimping & a pristine wire harness went bad but that is where the problem lay.