Changing Rear Speakers in a 1996 a4.

First remove the rear seat bottom cushion (grab the front edge roughly in the center of each seat position and pull up firmly, and it'll unclip and come out). Then fold down the rear seatbacks. Now, the side bolsters for the seats (between the seatbacks and the doors) have to come off. First, pull down strongly near the bottom (there's an upward-facing hook there that clips onto a ring and holds the bottom in place). Pull down and forward and it'll come off. Then slide your hand about 75% of the way up, and pop the bolster straight forward to pop out a retaining clip. Finally, you can remove the top of it from its mount, and you'll have the bolster out. Repeat for the other side.

Go into the trunk and look up at the bottom of the rear shelf. The harness plug for the third brake light is there - unplug it.

Now, back in the rear seat area, just grab the front edge of the parcel shelf and lift upward to pop it loose, and the whole thing will slide forward. Then you can remove your speakers.

Install is the reverse of disassembly.