i went to my local proaudio shop and they had a wiring
harness/faceplate in stock for my '96 A4, which should be the same for
your car. Mine also has the bose system, dunno if your car does.
also, it is VERY tricky and nonstandard to hook up an aftermarket CD
player to the bose system, you need a "floating ground adapter" that
converts your four pairs of +/- leads from the CD player to a 5 wire
setup: four positives and one common ground, because that is how the
bose head unit sends the signal to the amp in the trunk.
so just google search for "floating ground adapter" and whatever brand
comes up the most is the one that you use, all i remember is its a
little green box the size of a deck of cards, with wires out both ends
to splice to the new cd player and the dashboard wires.
personally, i would recommend just taking your car to a car audio
installer and have them do it, because it took me about 9 hours of
headache-inducing labor to finally get my cd player to even emit sound,
although i admit i had no idea what i was doing with the bose system to
begin with, it is NOT friendly with an aftermarket cd player, not at
all designed to be modified.
also you have to figure out which of the 20 or so wires plugged into
the bose head unit are the positives for the speaker signal and the
negative, and also you need to find the accessory turn-on wire for the
rear amp, otherwise you are screwed b/c the amp will not power the
speakers, hence no sound. Its worth the $40-80 it will cost to get the
cd player pro-installed, trust me. its the only time i will say this,
b/c i do a lot of stereo installs for friends, the bose system is just
plain stupid and a waste of time to mess with yourself.