check the fuse that's by the battery in the black plastic thing on the front of the strut tower, it's a 80 amp strap fuse and it can corrode and or crack due to age, if the bimmer dealer doesn't have one the mercedes dealer has them for glow plug relays on old diesels, ask for a 80 amp glow plug relay fuse for a 84 300d turbo if you need to go to mercedes