Installation of the CD changer in the A4 Avant is simple, but there's one good piece of advice I can offer. When you remove the cover on the left side of the cargo area, you see the space for the CD changer and the phone (both units mount in a metal rack). You want to remove the metal rack, mount the changer in the rack, then replace the rack.

I installed the standard Panasonic 6-disk unit. It came with plenty of mounting hardware in the box, plus an adapter packaged separately. The only hardware you need are 4 screws for attaching the changer to Avant's mounting rack.

So, in order:

1. Locate the three screws holding the mounting rack in place, shown by the three white arrows in picture 1. Remove these screws and slide the rack out of place.
2. The CD changer mounts in the middle of the rack, as shown in picture
3. Two screws go in each side, in the places shown by the white arrows.
4. Locate the CD cable in the well; it has a foam wrap on the end. Remove the wrap, then connect the cable to the adapter, and the adapter to the changer. You can also run a ground wire (included with the changer) from a tab on the changer to a grounding spot in the well. I didn't find any obviously good places.
5. Slide the rack back into place, keeping the cable pushed in behind the rack, and secure it with the 3 screws.

Whew. Tough job, but somebody had to do it. Put your favorite CDs in the cartridge and enjoy it. It will make you realize just how lame the FM tuner is.