I've a Blaupunkt in my 156, although mine was a 2005 model fitted with a single Cd player. You should have a tag that has the number to unlock your radio, it may provide a clue. My tag has a Blaupunkt reference. Try this web link and make sure this looks like your stereo.
It depends on the year of car up to 2000 they fitted alpine sets which had a little green display under the tape player about 2.5 inches long. Then after that they went to a single cd clarion. Hope that helps.
Not all Alpine changers work as they changed the protocol at some stage, the one above does work.
The newer alpine units use Ai-Net (model no beginning with cha), no way these will work. Older ones used a system I think it was M Bus (may be wrong, model number beginning with chm). I haven't tried any others from this vintage, but can't see why they wouldn't work.