Absolutely alternator brushes. You can replace then yourself for under $10, or replace the regulator itself.

You can remove the regulator from the (Bosch) alternator by removing 2 phillips-head screws. To replace the brushes you'll need a soldering iron ($15), solder ($3), and the new brushes ($5+shipping).

In the mean time, you won't be able to drive anywhere without sucking the power straight off your battery.

There's a possibility that it's a fault in the alternator itself, but you can verify that the brushes are worn out with a visual exam of them.

Happy Bricking
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1990 740 Turbo, on its way to stock specs, maybe beyond