Firstly, i don't see why German cars would need a dedicated workshop for minor items such as a compressor swap out.. I'd understand if it was a major engine, gearbox or electronics repair but there's nothing otherwise that a half decent mechanic couldn't fix... Not like the cars are made from materials from outer space...

Having said that, there's a shop next to oasis cars on the Sana signal that only works on mercs so if they can fix a merc, they can do a beemer too,., other than that, there's another BMW workshop on street 24 industrial area near popular auto parts... Try your luck with these and the other places suggested by others above... Best of luck...