Well, I'm still not sure what happened, but I am sure of one thing - I need a new harness. I got under the dash yesterday and found a lot of melted plastic. Both wires from the ignition switch were fried, and the gas gauge was welded to my brand new heat gauge. I'm guessing they were touching, and the gas gauge used the heat gauge for a ground and the truck turned itself into a toaster of sorts. Anyway, a major part of the harness is a plastic and copper souffle, so I ripped the whole thing out and I'll buy a decent harness with good instructions and start over.
What amazes me is that it took days to put the thing in, but only about ten minutes to rip it all out.
The fuel gauge may be okay with a new decal, but I'm leaning toward replacing it anyway. The sending unit is okay because I didn't have the wiring back that far yet, so it wasn't invited to the cookout.
Could the simple thing of the two gauges touching have caused this mess? Could there have been a missing insulator between the fuel gauge and the gauge cluster housing? I notice there appears to be insulators under the other gauges, but not the fuel gauge.
Where's the best place to get a good harness with idiot-proof instructions?
