This tail light assembly has two dual element lamps (3057) and a brown, green, and two white wires to it. One white is ground, one white is 11.2 VDC to backup light, green is 11.7 VDC to brake and turn signal light and brown is 10.6 VDC to park lights so these are dim. I checked harness under vehicle and still just 10.6 VDC on brown. Seems like a broken wire would yield 0 volts. What problem would yield 10.6 volts?
tail light problem on 2000 Chevrolet Express 3500
Do you have a full 12.6 volts at the battery? If you do it sounds like a ground issue. Make sure all grounds associated with these wires are clean (corrosion free) and making good contact (tight).
I do have 12.6 volts at the battery. I suspected the ground also, so all my measurements are to battery negative post, not a ground wire in the harness. So the problem must be on the positive side of the circuits.