Definitely yikes... I would not let Circuit City or any installer like that touch any of my cars, especially the Porsche. It's so easy with just buying a snap-in harness and doing the install yourself. By the way, how's your FM radio performance? I have had several VWs and they require an antenna booster because if not the radio (particularly AM)
That said, your problem sounds like alternator whine, which has been said here is due to crappy grounds. You could try the internal radio amp, but I wonder whether this unit even has amplified outputs - a lot of the naviagtion units don't because so many cars have built-in amplifiers anyway or people also have upgraded audio systems. Also, you have to go through the car's stock amp to be able to use the built-in speakers. If you are replacing the whole system, then run a new amp and speakers with good wire and bypass the stock amp.
As far as the grounding problem, Radio Shack sells a little device called a ground-loop isolator. That will significantly cut down on ground-induced noise, so take a look at that - it's pretty cheap.
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