I am absolutely certain the case isn't supposed to be hot. It's either connected backwards or it has no ground and you are getting arcing to ground when you bump it against a chassis part. If it were connected backwards it would run backwards, and would blow a fuse when you grounded the motor case. Since it hasn't blown a fuse, it tells me your - (minus) side of the motor isn't connected to a ground.
Quotes to live by:
guessing gets expensive
nothing is more expensive than a cheap BMW
buying a ratty example (of a BMW) is a parasitic relationship.(and you ain't the mosquito)
Ken Kanne, Silverhill, AL, E36 Forum Mod/Craigslist addict/Hoarder of all sorts of stuff
1995 318is "Bebe"; 1993 325is "Elvira"; 1985 635CSi "Katja"; 1984 633CSi "Sylvia"; 1987 325is "Odette"