I have a 1997 850 that also has a mysterious battery drain. Was lasting about a week before battery dead. After about 6 months and 2 battery's I finally got serious about it today and checked the drain with an ammeter between the positive battery cable and positive terminal. It was about 170 Ma. I started pulling suspect fuses and found fuse #15 (OBD, courtesy lights, door open warning light, remote central locking system)to be the majority of the parasite load,(about 150Ma). During my investigation of the interior lights, I noticed that the black pin of the glove box switch was missing. Turns out the switch itself fell out of it's bracket under the glovebox liner and was somewhere inside the dash, always on. I had to take the glovebox liner out to get to the switch. Rather than find a new mount for the switch, I merely removed the bulb from the socket. After removing the bulb, my load on fuse #15 was gone. Hope this helps someone.