I'm in the UK and have a 5 series which came with a stock headunit, slot in cd in dash and cd changer in the boot. I ripped all that out and fitted my RioCar since I have pretty much abandoned CDs now but the conversion facia is ugly.
I have been in touch with a few guys on the cardomain web page and it seems several of them have managed to get DIN sized stereos "professionally" fitted into the air vents above the stereo. I'm not sure what the layout of the 3-series vents is, but if it's similar to the 5-series and you don't really want air from the centre console then perhaps that could be a consideration.
I've just had a look to try and find some pics for you but seem to have lost the links but if i find them again i'll post them for you.

Scoopz