It could be as simple as the small red wire loose, corroded, or fallen off of the alternator. It is the "D+" wire or exciter wire, connected to the instrument cluster warning light(s) usually with a spade lug.

It could be a broken ground wire, a short heavy-gauge wire with blue insulation.

Or, the brushes have worn short, and a replacement set is in order. The regulator and brushes come as a module (the black object held by two screws to the back of the alternator below) to be replaced together, but if you're handy and cheap, you can solder in new brushes alone.






--
Art Benstein near Baltimore

When an actress saw her first strands of gray hair she thought she'd dye.