I have a 2004 Boxster with the premium Bose sound system as well. The car came with a Becker CDR-23, which left me with very few options.
- Install a Dension Gateway. This is a neat device that I hear provides excellent sound quality. You can Google Dension Gateway 500 to see where to buy them and some DIY install instructions on various forums. The downside is that if you don't already have a CD changer, you have to have the CD changer option enabled by the dealer. The Porsche dealer nearby wanted $135 just to "try". I hate giving those guys money. My independent Porsche mechanic said he couldn't do it without the proper lock codes and recommended the dealer.
- Replace your entire audio system (including speakers and amp). Expensive and it seems like a waste to me, but I have a friend who did this. I haven't verified how much of the existing audio system actually can be reused, but I heard none of it.
- Get an FM Transmitter, something like this: Newegg.com - MOTOROLA T505 Bluetooth In-Car Speakerphone with Digital FM Transmitter Unfortunately, sound quality isn't very good.
I personnally think the the Dension option is the way to go, but havent pulled the trigger yet.