Not sure if it's related, but when I bought my 77 it had no dash or parking lights.  When I started tracing wires I found that the previous owner had used the dash light power for a ground on the radio.  When the lights are off it reads as a ground so I see where the mistake was made, however, instead of figuring it out the right way he cut wires and rerouted everything to make it a ground for the radio to keep from blowing the fuse.  I had to do some wire repair work, but eventually I got it all figured out and wired in properly.  So, moral of the story, make sure the radio is grounded into the dash lighting.