About the headunit, I my self have changed H/U in a older ('95) A8 4.2Q (bose and quattro was standard on these for those who didn't know
) the H/u swap is quite easy, but if th C4 is having the same piece of junk stock amplifier then your screwed... Every time the amp cuts the voltage from the preout you'll hear a big ugly POP, from all speakers. They will still work and play music, but I would either change the speakers and amp, use a high-level/low-level converter (havn't tried it my self, but it might work), live with the pop in every track change, power off, power on and so on, or change the amp.