Well, today the weather was finally warm enough to pull the rear door card off and see what was causing the short that made my brake/tailights inoperable when the door was open, and intermittent when the door was closed. I thought it would be easy. It was not.
First but minor concern is there is a bit more rust than I'd hoped. Looks like a new door, or at least a door frame is in my future someday. But I have some time.
There are basically three "harnesses" in there. One goes to the third brake light, one to the window defogger, and one to the wiper motor. And one big black wire that seems to serve as a ground. To this, the ground for the wiper motor are spliced in. Not exactly how I would have done it, but oh well. That's the way Solihull did it.
I pulled the ground wire from the door frame, cleaned it, used some light sanding, bright shiny metal. That didn't do it. I noticed the screw would not fully tighten (yes, I have a screw loose), so I drilled a new hole. This still did not work. I did some measuring, and noticed low resistance between the loop and vehicle body. Just to be safe, I cut the wire back to before the wiper motor was spliced in to cut out corrosion, and added a new marine grade loop, and gave the wiper motor grounds their own loop. All screwed tight to bright metal. No Joy.
Started wiggling wires. Noticed something odd:
Lights went out when the door was open, but worked when the door was closed.
When the lights were working, the third brake light had the faintest glow. It lit up brightly when you pressed the brake, but had just a hint of volts at all times when the headlights were on.
Disconnect the lightbulb, no brake lights. Reconnect, door closed, it worked fine (other than the faint glow).
So I think the problem is in the third brake light harness. It could be the connectors, or it could be the wire. It was getting dark, and I was getting cold, so I put it all back together. I know I'll be in there again. But in the meantime, I think I can get it to work when I need.
Thoughts/ideas/comments on electrickery are appreciated.