Digging through the factory diagrams it looks like all in-cab electrical stuff grounds through the body of the healight switch.

I fiddled around with it this weekend. The fuse box has three buss bars in it. One of them runs the door locks and A/C. Both are high amperage consumers. There was enough voltage at the bus to turn on a trouble light.
I pulled the fuse (circuit breaker actually) for the windows and jumped power from another bus bar to the output side of the fuse. The windows worked fine and did not turn on the trouble lights.
This tells me that it's an amperage supply issue and not a ground.
I will have to go through the power supply and bulkhead connector to look for issues. I'm strongly considering replacing the fusible links with fuses.

I guess I'll go through everything while the dash is out. Put the blower motor on a relay, replace the fusible links with fuses, and improve the dash lighting.