Do you want me to scan and send you some diagrams from my '89 factory manual?

One thing that I can think of is that there may be an insulation break down that only appears when you put a load to the wiring. Voltage passes OK to the meter because there is no current draw, but as soon as you plug in a resistive load, the faulty insulation shorts to ground. I've seen it happen before and is a [censored] to deal with when you don't know it's there. You can diagnose it easily if you can get your hands on a megger, though. Just isolate the wiring you're testing, apply the 500VDC to the wire and get the resistance reading. You'll burn up 12V components at first contact, though, if you don't isolate the wiring properly.

HTH