1997 GMC Sierra no tail lights

I have a 97 gmc sierra c1500. One day my taillights just stopped working. My brake lights, turn signals, hazards, and backup lights work fine. All other lights on the vehichle work fine EXCEPT the taillights and license plate light (which is on the same circuit). I replaced ALL the bulbs, replaced BOTH circuit boards, checked all the fuses, not just the ones for lights, and replaced the headlight/dimer switch assembly. Heres the part that has me stumped. There is a 4 pin connector in the rear that connects to the rear harness. The brown wire is the wire for the taillights/plate light. If I disconnect BOTH taillight assemblies I get full battery power at the plug on both sides. If I hook up EITHER taillight assembly, the voltage reading drops to zero. As soon as I unplug it. Voltage goes back up to full batt voltage. I tested this at each taillight plug. At the 4 pin connector that the whole rear harness hooks up to from the front. Same results. Full batt until I hook up one of the taillights. I tried running a jumper wire from the 4pin directly to the taillight assembly to bypass the entire rear harness. Still drops to zero. I tested it with the meter negative lead on the ground connector of the harness AND on the frame ground where the ground wire bolts to the frame. I took off all the ground wires, cleaned the surface, reconnected, still same results. I tried putting the old circuit boards back in, same issue. I am completely lost at this point. I am getting full bat volt at the switch. Please. Any help you can offer is appreciated.