You see this problem a lot on the brickboard. When it happened on my '91, I don't recall if it was the regulator or brushes or both, but I remember that it was only a few dollars for the part (less than $20) a plastic assembly on the back of the alternator, and I put it on in 2 minutes with just a screwdriver.

As always, before spending any money on the alternator, check that it is well grounded, and check the insulation on the red exciter wire where it passes around the bracket.