Update - I crawled under the truck to start poking around. Looks like someone has already replaced part of the wiring harness. The wire is housed in black plastic corrugated loom, the kind with the split up the center. There's 4 wires, one labelled "left signal", "right signal", "running light" and one that is labelled "fuel gauge", so mystery solved, at least for the time being.
Any idea what color the brake light wire is supposed to be?
Chris
