The audio board has a couple of large dynavin threads
Quite extensive ones even showing how to install
The problem with putting any aftermarket head in is you loose
BMW Assist , bluetooth , ipod and break the Most Bus.
And the major one is that you can't get BMW software updates
after messing with the Most Bus and Can Bus
Some people mount there original head unit somewhere else in the
car to overcome this but its not an elegant solution.
http://www.e90post.com/forums/forumdisplay.php?f=27