I've been tracing down a pretty significant voltage drop on my volt meter that occurs any time I switch certain loads on, such as the blower, radio, etc. To rule out a faulty volt meter on my aftermarket gauge cluster, I measured the voltage at the control box for my aftermarket gauge cluster, as well as the cigarette lighter port, and I still noticed a pretty significant voltage drop (anywhere from 0-1v). I measured the voltage directly at my battery, and it stayed pretty constant around...

1985 GT significant voltage drop with certain accessories