The 2000 S4 CD changer is mounted in the drivers side trunk area smugglers box, under the tool box, the mounting bracket is pre installed from the factory and does triple duty, the tool box on top, the CD changer underneath it, and space for other electrics underneath that.
1) The installation instructions which come with the oem Matsushita/Panasonic changer model # 4BO 035 11A 6 disk changer are wrong with respect to the H or V settings. Use H for 2000 S4 (I believe this to be true for late A4 also).
2) Remove the tray on the right side of the trunk by lifting it straight up. It is held in place by vertical plastic slots.
3) Remove the 8 Philips slot screws holding the plastic rear trunk liner in place.
4) Remove the rear trunk liner; lifting the rear edge of the trunk carpet will help.
5) The left side trunk liner must be partially removed, there is 1 plastic button screw on the aft wall. Turn to remove; it will be difficult to turn because it is hard to grip and has no screwdriver slots. The only thing holding the left side liner is now some clips which can gently be pried loose by hand. The panel does not have to be completely removed, only the rear half needs to be moved away from the body. The carpet should be partially lifted to facilitate this.
6) Remove the toolbox/CD Changer mount.
A) There are 5 - 8 mm bolts which can now be reached with a 1/4" ratchet and a 6" extension (a wobbly connector will be very helpful!). The forward most one is horizontal and only needs to be loosened, as the mount is slotted in that spot. There are 2 more vertical ones at the feet of the mount, and 2 more horizontal ones going into the aft wall/light box of the trunk.
B) The mount can be rotated out through the access door, to the smugglers box.
7) Again be sure the settings on both sides of the changer are H, it should come from the factory like this. Mount with 4 M5x7 screws.
8) Hook up the DIN connector protected by foam which can be seen in that area. I slid the foam back over the connection afterwards for a little bit of extra protection.
9) Button everything up, test.
