Audi Repair: electrical mess, aftermarket radio, independent problems

Question
I have a 94 100cs with 168K miles that is having an electrical meltdown -
hopefully not literally.  It started with the sunroof opening on its own, then
has progressed to the door locks, the security system (we must now "trick"
the car by actually locking the doors when they are open so that the alarm
thinks its un-locked and vise-versa or the alarm sounds or the car will not
start), and just recently, the radio/stereo can't be turned up because turning
the knob any direction shuts the radio/stereo off.  Our youngest daughter
drives it because its safe, but it's rapidly becoming a comic routine to drive it.  
The fuses have been checked and are fine.  Is this a systemic problem with
the entire system which would be too costly or would it be worth an
investigatory trip to our mechanic?

Answer
It sounds like you have multiple problems.  The stereo I suspect has a unique problem with the on/off switch-volume control.  These are normally soldered on the circuit board and the solder sometimes lets go.  To fix it you have to get to the board that contains the switch and typically re-solder the connections.  The sunroof control switch is a fairly high frequency failure item, so suspect the sunroof control siwtch, and the alarm/central locking module is also prone to aging as well as water damage because of it's location under one of the front seats.  So it sounds like you have three independent problems, very expensive to get fixed by the dealer but parts can be obtained used.  The radio can be repaired but it may be cheaper to put an aftermarket radio in.  Let me know how you get on,  Jan