Bill is right on, but if you are still running the original style, externally regulated alternator, I would start there before pulling fuses. My experience with this when these cars were still around is that a diode would short out in the alt and ground out. You can look at the dash in the dark and see if the alt light is faintly glowing too. I've seen that happen with a shorted alternator.
If the alt isn't the issue, then start pulling fuses...
