I've run into the same problem here recently after a large rainstorm. If the car will stay running while you've got the negative post off the battery then you've got a regulator problem. You can open up the casing on your voltage regulator and have someone start the car for you while you watch the Armature side of your regulator points. Once your generator starts charging it should close the points and shut the Gen light off. If not you can order a new one online from
AutoZone.com - Get in the Zone! for $32.00 shipping included stateside. If you find that your generator is shot and want to stick with the original setup you can have yours rebuilt or used as a core for a rebuilt generator from
C.P. GENERATOR INC. SINCE 1962. They've been the most reasonable I've found around for pricing. If you don't care about original looks you might want to look into a one-wire alternator setup. I don't have any links for this setup right off hand but you might find something on
Classic Inlines Performance Parts - Home. Hope this helps.