So, a black ground wire that showed poor connection now shows good? What could cause that? Sounds like signs of an advanced degraded/corroded 30-40 year old wiring system. In the headlight shell pic that shows the bent turn signal stalk, the ground lug on that stalk looks corroded. And, if you look at the way the lug is mounted on that stalk, you'll notice that grounding that stalk depends on pressure from the stalk nut, which cannot properly apply that pressure while in that bent condition.
I know that you're looking for a single-point-of-failure, it's probably there, but if the wiring system is in this shape, exhibiting unpredictable changes, that's a sign that the entire system needs to be thoroughly inspected and all connectors cleaned, known problem components replaced.
There's plenty of schematics here, and many threads about chasing electrical gremlins. If you feel this is beyond you, don't feel alone, we've all been there. You've just entered the twilight zone.