+1even installing speakers can be difficult... For example, with the bose system in my a4, the speakers are low resistance bi-amplified speakers, you can't just swap the drivers out with whatever you want.
Not to mention, i used to work at a place that installed stereo gear in cars. You don't want those idiots touching your car. Want loose door panels with stripped screws? Want screws and other parts rattling around in your doors? Unless you're going to do the work yourself or sit there in a lawn chair watching the guy do the work you are potentially going to get your brand new car back in worse shape than it was in when you handed it to them.