ugh not this again. Let me put this simply, The wiring system that is used in the newer 9-3's being post 2003 has royally screwed any major attempts of adding anything to the system. Saab had the brilliant idea of using mostnet. Thus said you can not replace the headunit per say. If you remove the factory head unit the car will not start. The mostnet system is basicly just data, if you break the system the car will not start. The only way to get an aftermarket headunit would to be 1) fresh wiring of the car. or 2) add a second head unit somewhere in the car or push the existing one back into the dash. Both can be costly, The door speakers, and dash speakers can be upgraded. The parcel shelf(rear deck) is tricky. They are wired directly to the headunit. I know all of this because I myself have a 03 9-3 and have been fighting with it to somehow add aftermarket speakers into it.
Any questions?