You will require an AMI interface out of an A4/A6/A8 - Check part number first, most start with 4E0 which is perfect.
You will need:
4D0 051 530 - Release Tools to remove CD Changer (Approx £5)
4F0 051 510 K - iPod Cable to fit AMI (Approx £20)
4F0 051 592 A - Wiring Harness for Power and Retrofit Kit - (Approx £30)
You will not need the last one if you simply replace the CD changer. I wanted both
So that is the official audi dealer retrofit kit number. Release tools make it easy.
Basically drop glovebox, remove the CD Changer first, then fit the wiring harness to split the MOST bus and power for the AMI most, and fit both.
You will require the latest MMI software installed. Let me know if you need a copy. If the AMI is new it will require coding, if not it should work immediately
Done