I'm going to suggest this is the brush pack, but before you go to that trouble (and it comes out with two screws--alternator in place) I recommend you check the alternator ground strap. Since the alt is mounted in rubber or urethane bushings, there is a heavy ground wire that runs from one of the alternator through bolts, or a screw on the back of the alternator, to one of the mounting bolts on the alternator base where it bolts to the block. Also check the small wire on the back of the alternator for breaks. Craziness in the instrument lights is a common problem when the system is not getting a charge to the battery. One more thing: With your engine off, and the key on, make sure your alternator light in the cluster illuminates. It's part of the exciter circuit.

DS