I know that for cars with a nav system, you can upgrade the nav compter to Mark IV, upgrade the radio in the trunk, and add a BMW factory Siruis system. DOn't know if that's possible for cars that only have a tape deck.
There are a few resources out there, check out http://www.bimmernav.com/page6.html
, it may not apply directly to you, but might get you closer - and another set of possible contacts...
Hope it helps (and doesn't send you on a wild goose chase...)