I'd go with the Alpine CHA-S634 6 disc changer, Blitzsafe
BMW/ALP DMXV.1 96, and the Alpine KCA-130B. (I don't think it will work
with DSP though)

I got mine from Crutchfield and it worked great.  I got the bracket to
hold the changer from ebay for like $25 brand new and the alpine is a
direct bolt in.

The nice thing is it's a 6 CD changer that works with your built in
head unit and it also plays MP3s!  So you can get 100+ tracks per CD vs
20-25 on a normal CD.

I keep my tracks limited to fewer than 99 since the head unit will only
display 2 digits.

One weird thing I found is that you need to make sure you have a
regular CD in slot 1.  For some reason if mine has an MP3 in slot 1 it
takes forever to start playing the music (like 20 minutes!)  If there
is a regular CD in slot 1 and the rest are MP3s it plays fine.  It may
just be my CD changer (I need to work with Crutchfield on it ) but I
doubt it.  

Jim