Apart from regulator failure, the usual suspects are the two yellow wires feeding the charge from the alternator to the reg.
If you take th ebattery trya off, there's two connectors to the reg. One is a 4 way jobbie, the other a two way. It's this one that connects the yellow wires.
Check the wires and connector. They've probably melted.
Cut the connector out, cut the wires back to good and fit bullet connectors. Wrap them in tape to keep the muck out.
You can solder them but this makes servicing a pain as you can't remove the tray out of the way
