My initial description may have been lacking. I'll try to clarify.
The rear lighting harness is disconnected at the factory plug just before the rear bumper. When I apply power to the other 3 wires in the plug, the associated light comes on. However, when I apply power to the dark brown taillight wire, it trips the breaker on my power probe (when the harness was plugged in, it blew the fuse).
There is no visible damage to the harness. The trailer lighting was a factory-style plug and play setup (no spliced wires) but I removed it from the circuit to eliminate it as a possible problem.
This is a power probe. One of the most useful tools I ever bought.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1