In my experience, most electrical problems can be traced to a bad ground. Your battery needs to be grounded to the engine, the frame and the coach of the car. The car is mounted on rubber and needs a dedicated ground. Just because you have a ground cable, don't assume it's good. put a volt meter between the battery and the other end of the ground cable. Positive cable of the meter to the ground of the battery and negative end where the battery ground connects to the car. You are checking for line loss in the cable. It can't be the ground cable, it's there! and has been for 60 YEARS.
I forgot, have someone try to turn the engine over while checking. Should be very little voltage.
The car needs a good ground to operate everything on the dash. A friend had a Jensen Healey car that had a very weird gauge cluster. The blinker lights would come on intermittently, the radio would work and then not work, the alternator light would glimmer weakly. He had it to several shops, no help. He had radio electronics mechanics from United Air Lines try all to no avail. I put a test light between the master cylinder mounting screws ( a good ground ) and his dash cluster and got a light. Made a dedicated ground wire and connected to the dash and no more problems. My father had a car that would not turn over. New battery, no help. Jump the car with pos to pos and ground to alternator mount and it would fire right up! Bad ground cable. Not everyone will check a ground because they see it's there. I am not god of the mechanic's, but I did make my living since 1970 working as one. Just trying to help, good luck.
