There has been a thread on this subject already. Here is some of the banter:
http://board.marlincrawler.com/index.php?action=search2Check the obvious, fuses, loose connection at the alt, check all grounds including the grounds at the engine and frame, take them off and clean them.
I had the same problem on my 83, turned out to be the main ground on the drivers side, mine was wrapped up with the alternator wires.
