Could it be a ground ? Follow negative battery cable, and add 10 guage
wire from main ground to radiator support, and another to firewall, and a
third to fender that
has heater blower [ fiberglass box ] grounded to it.
Be sure all metal connections are clean, paint over for corrossion
protection. Good grounds prolong the life of electronic components. On
frame cars, I like to add a Ford Starter cable from engine block to frame