If playing with the negative battery cable ultimately fixed the problem.?????
I would get a battery post cleaner and clean both posts and cable ends.
Also I would clean the connection of the negative cable where it bolts to the body. make sure there is bright clean metal where it bolts to the body. if it is on a stud held with a nut wire brush hell out of that stud.
Most electrical problems are first: bad grounds Second: corroded connections. really bad crap third and forth.
Glad you found the problem. Thanks for letting us know the fix, helps us all learn.
