Nice find! If you want, I'd be happy to check it out for you. In fact, I'll be in the west end on Tuesday inspecting an A3 for a friend.
I looked into the Dice kit too. Very cool, but it started adding up fast with the DSP and NAV kits, which the M5 has both! Instead, I went ghetto and did this... If this were my daily driver, I might have done it.
E39 Aux In
This site has a similar write up too, but they added a preamp to boost the volume. Look for "MP3/aux input install" under the Stereo section.