First sentence of post number 10.
Grounds are horrible in these old girls, and I use a separate grounded wire to test this.
Hook it to your bumper or wherever you can get a good ground, then touch the base of the offending light with the other end of your wire while the light should be on.
If it lights when you touch the bulb base you found the problem.
Touch the actual base metal of the bulb, and not the socket because the socket can loose it's connection to the lens housing.
I would do this before I went through a bunch of possible pointless diagram hunting.
