I really doubt that it isn't part of the problem. Been burnt by them enough times to give them NO credit with reman electrical parts. None.
Yeah, burnt wires under the dash would be the first thing that I'd investigate. Then I'd wire around the ammeter so that it and the firewall connectors aren't a fire hazard. Check/replace the battery. If all of that is good, quit wasting time with the big chain stores and go to someone who knows wtf they're doing.
