Have you looked in the ICE forum? There's lots of info there from people much better qualified than me, but here's a rough guide to be going on with.
The standard Audi H/U only has line-level amplified outputs for the front door speakers. The rear speaker are active, i.e. they are fed by non-amplified pre-outs on the H/U, and have their own amplifier built into one of the rears.
That means you need an adapter to connect the active rear speakers to a pre-out on the after market H/U. It also mean that you will get a huge disparity between the sound level to the front speakers that are powered by the H/U amp (typically 2 x 50 watts these days, even on an entry-level model), and the rear speakers that are "powered" (& I use that word carefully) by the puny 25 watt amp bolted to the speakers.
This means you have to bias the front/rear fader heavily towards the rears to balance out the sound. It also means you're having to suppress the output from the H/U's good quality amplifier so it doesn't overwhelm the output of the shitey rear-speaker amp, which kind of defeats the purpose. It is possible to disconnect the rear speaker amp, connect the cables direct to the speakers & then connect the other end to the line-level rear speaker outs on the H/U. A better solution, however, is to throw out the rear speakers in the bin with the H/U (they are made by Nokia, for God's sake!) , and put in some proper ones.
Also, you will probably find you need an in-line aerial amplifier sitting between the aerial cable and the aerial socket on the back of the H/U. This is because the rear-window aerial has to be amplified, and on most B5s, the aerial amplifier is built into the original H/U itself.
Finally, you'll need fascia adapters to fill the gap either side of the new H/U. as the outgoing Audi unit is wider than the one you are replacing it with.
That's the non-technical answer. Over to Andymac and the others for the science bit. /ubbthreads/images/graemlins/beerchug.gif