Brimech,
I did some research ( what can I say, slow morning at work ) and unfortunately I could not find a solution for you. The Garmin 660 does come with an line out jack, so my first thought is rather than going through the FM transmitter you could plug directly into the radio. But, since you have a 2003, I believe you have the CDR23 radio, which does not have an AUX input available :
http://www.beckerautosound.com/Porsche/Porsche%20Menu.html
Sorry,
Nick