Hi all,
When I got my A6 2006 Avant it had the 6 CD changer in the glove box. I had bluetooth function for the phone (if you had one of the shark fin aerials on the roof I believe that generally means you have bluetooth connectivity) but doesn't have media functionality, you can just make calls.
I bought an AMI unit from eBay, (part no. 4E0 035 785 B), and replaced the CD changer - took 5 minutes. (I had already updated the MMI software to the latest versions with CDs I burned from the internet).
I then bought a Tune2Air bluetooth device from ebay. With an Audi AMI ipod cable, this plugs directly into the AMI unit, streams the music from your phone/ipod/whatever (iTunes, Spotify etc) through the stereo. Tracklisting with appear on the MMI screen for most devices, and tracks can be skipped with the < and > buttons.
All works perfectly, probably about £500 in total to do the work.