Taillight wiring isn't exactly complicated*. If the 20+ year old wires are having problems due to age, why would a junkyard replacement be much better? They're just wires. If you have replacement wires, you can "build" your own "harness" pretty quickly and easily. That's how they do it at the factory, so you're not really "butchering" anything. I put "build" and "harness" in quotes because the terms have to be used VERY loosely when talking about something as simple as taillight wiring.
*The wiring on these trucks isn't complicated, period. With the right spools of wire to get the colors right, you could likely rewire the whole thing in a few hours if necessary. No fuel injection, no ignition, barely any electric accessories. If there's a problem, just run new wire where necessary and be done with it. Cheaper, easier and faster than trying to locate a "replacement harness." If the trucks were more modern and actually had complicated wiring, I could understand it, but not for something simple.