If you're searching for used parts to fix this beast, try PSN (PowerSportsNation) out of Norfolk, NE.
They are a big outfit that specialize in parting out quads & UTV's, like 30+ machines a week.
I can't think how it would/could cause it, but I think I'd suspect the neutral light switch. I'd test that idea by by-passing the switch with a piece of wire I could earth out. If it worked normally with the wire earthed then I'd figure the switch was not making proper contact.