I'm seeing this behaviour too. Turning on the parking lights drops 14->13, headlights take it 13->12. Time to clean the grounds? I just replaced the belt yesterday after it came apart while my mom was driving my car. (only lasted 2 yrs. but the dayco it replaced I kept as a spare and still looks good).
Is the battery also a source for issues in this case?
Or primarily the grounding points?
I have had my alternator rebuilt in the past year (bearings nearly seized up) and the regulator replaced 6 mo. before that. And I resoldered the headlight relays a year or two ago. I also replaced all my accessory bushings two years ago (5 total I think)

thanks,
Will
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1990 740 Turbo, on its way to stock specs, maybe beyond