take off all the battery, starter and alternator cabling, clean up all the connections till shiny and refit.
if its an early car the battery earth goes to a studf on the gearbox upper bracket, which is a poor place to earth it. the later cars have a different rear gearbox->engine mounting bolt with a stud on the end, and battery earths there instead. recommended to upgrade to this setup

ideally get uprated cabling, MK3 +ve wiring fits straight on but -ve doesnt quite reach the earth stud on the battery tray. B4 passats sometimes have a cable that fits straight on though, but you could just make up a short length o chunky wire between the MK3 cable and the battery tray stud.