Well I feel right stupid. I spent hours of my weekend chasing an electical issue...
I noticed that my front parking lamp wasn't working, and since that's where my aux water temp and oil pressure gauges are wired into, I had to track down the source of the failure.
So I cut all the old, mangled wiring out and added a nice twisted pair for power and ground to each gauge, and the gauge lighting. I turned the key on, and patted myself on the back when I got 10v (I chalked this up to a low battery) at each terminal).
I put a parking lamp bulb in, and...nothing. I put a tiny tic-tac bulb for the gauge lighting in, and...nothing.
Wtf?
Pull the meter back out, and I can only read .2vdc at the bulb, with the bulb plugged in. After some headscratching, I noticed that I had +10v with the bulb out, and .2v with the bulb (any bulb, parking or gauge lamp) plugged in.
I thought that was strange, but everything else seemed to work just fine.
I grabbed my tail section and plugged that in...the brake light worked, the turn signals worked, but the rear running light did not. Strange.
I noticed that circuit was doing the EXACT same thing as the circuit at the front...10v with no bulb, dropped to .2v with a bulb plugged in.
I went to my wiring diagram and started looking...everything led back to the Bulb Monitor Unit. I thought that must be going out, but before spending $50 on a replacement, I found bypass instructions on the forum.
Tried that, no change. Damn.
Wired everthing back up, and went to bed for the night. I sat there for an hour rolling over in my head why these damn lights weren't working...
Came back the next morning, poked around a little more. Briefly considered rewiring the front and rear lights to just bypass my issue, then snapped out of it.
"It has to be something stupid" I said to myself. "What haven't I checked?"
The fuses.
No. It can't be a blown fuse. Can't be.
Guess what it was...yep, a popped #2 fuse. All that fussing and head-beating, and it was the simplest possible answer.
I replaced the 7.5 amp fuse with a 5amp, and it seems happy thus far. The PO had wired in a large, three lamp rear running light. I suspect the fuse was blown when he did that, which explains the extra relay just laying in the tailsection that took unfused power directly from the battery. That's all gone now, and we're back to a functioning, stock electrical system.