40 year old wires

Definitely a case of demonic possession in those 40 year old wires. :) Been there with my 66 Valiant over the past 28 years, and at times you just want to try an exorcism, or maybe a match or small hand grenade!! But if you follow the wiring diagrams and just use a logic tree mentality, you'll eventually get to the root of the problem(s). One place to check closely is the blocks where they join on the inside/outside of the firewall since these get dirty and corroded over the years and sometimes they get physically damaged but not visibly so unless you break them apart and do a close inspection of the connectors. Good luck and keep us posted on your progress.

Sid