Check these out:

Make sure harness bulkhead is seated, not loose, ammeter lugs aren't loose, switch connector, short in the alternator.

56 Plaza 63 D100 step side 67 Coronet, 68 Roadrunner, 69 Super Bees, 69 Coronet 500 convertible, 70 Roadrunner Post, 79 D150 360, and a severe case of Mopar a,d,d