Perhaps time to inspect the leads connecting the the 12V
fuse panel. On your fuse panel, which, if your '79 is like my '74, you will find attached to the face of the Univolt, look carefully at the right side wires coming into the panel.
The two upper ones on the right (purple and yellow) side may be loose, corroded, or both.
If such is the case, cleaning and tightening will be a temporary fix, because what could be happening is that the metal of the terminals is increasing in resistance as it loses it's temper.
If I'm right, change the fusebox, and while you're at it, the Univolt, too. You will be pleased to not hear the hum it likely makes any more, as a side benefit.
[Edit: all bets are off if it's a fluorescent, all my internal lighting is regular bulbs]