[JimD]

The attached file is a good summary of 1 series audio options through 2011. I think 2012 and 2013 have the same options as 2011. Note that the base system is different for 2008/2009 versus the later models.

But the top level system, for all years, has the signals converted to optical. That could make aftermarket systems significantly more complicated. That technology is common to other bimmers, however, so good input may be available on the 3 series forum (or others).