I have wanted a way to stream Bluetooth audio to my existing CDR23 system while keeping everything stock looking. This would let me use Android Auto on my Samsung Galaxy 8 Plus to control the audio source, navigation, etc. and utilize the existing CDR23 headunit and stock amplifier. It looks like the exact system I was looking for is now available.
I just installed a Mr 12volt Bluetooth A2DP USB MP3 adapter for Porsche CDR23 CDR24 Boxster Cayman Cayenne (search words on ebay) in my '03 Boxster S. Note that I have NO affliation with the manufacturer or supplier and paid the full $155 retail price shipped. Installation was very straightforward. It just requires power, which I got from the existing phone pre-wire (red is Vbat and brown is ground) behind the center console. The system was supplied with a MOST bus loop connecter, which was perfect to loop out the CD changer, and a MOST bus inline adapter that is connected in series with the headunit and amplifier to get the Bluetooth adapter into the MOST loop. This does require removing and reinstalling the headunit for installation, so you will need radio tools to release it from the dash. Those are not supplied with the kit, so make sure you get those.
I already had the CD changer, which I removed, so no programming of the car was required. If I didn't have the CD changer, I would have had to enable it with a Porsche diagnostic tool, like the PST-2 or one of the PIWIS systems.
This does come with a microphone for the phone functionality. I disassembled the included microphone and installed the element in the factory location in the dash. This is the cover with the built in grill that you remove to access the left cluster screw. I did put a small connector (a connector normally used for RC servos) to make it so that I could remove the cluster without disturbing the microphone wiring, pretty as it would have been if you got the car with the phone system. You definitely don't have to do this, by any means, but I like that there is absolutely no trace of the system once it is installed this way. The Bluetooth adapter itself is small enough that it can be tucked behind the center console, so it is out of site, yet you can get at it if you want to connect the the USB or Aux In cable without too much trouble.
I have my phone set up so that it automatically goes to Android Auto once the phone connects to the Bluetooth adapter. To switch to the Bluetooth adapter I just select CDC on the headunit and audio is now coming from the phone. I think that you could control the tracks, etc. from the headunit, but that part is not intuitive and much easier on the phone anyway. Android Auto gives just the controls you need when driving, but is not necessary. I imagine Apple CarPlay would work the same way.
On my car I have the GT3 center console delete, which makes a perfect place to put the phone in landscape orienatation. The last stage of this upgrade is to put a wireless charge pad behind the carpet and a custom bracket to hold the phone. This will then put the phone in the perfect spot to be visible for navigation and close enough to control, yet it will be off of the dash and out of the sun with the top down. Power for the charger will come from the terminal 15 wire on original phone pre-wire plug, so that the charger will only work with the ignition on. As it is, the Bluetooth adapter "wakes up" with the headunit, so if you turn it on with the key off, the Bluetooth will work as well, just as it should. The Bluetooth unit also has USB, Aux In and SD card inputs. I could use the USB for charging or direct connection, but I would rather be cordless.
All in all, if I sound a little excited about this upgrade, that is because I am. This gave me all the functionality that I was looking for, without the need of going to an aftermarket headunit and rewiring the system, or putting in a double DIN radio and having to move the climate control head. Years ago I worked at a car stereo installation shop while in college and I had my share of crazy sound systems. Now I tend to like to keep things stock appearing at least. I think the last sound system upgrade for system will be replacing the stock dash and door speakers, and converting the Bose rear speaker that I got to "normal" impedance car speakers.
Bottom line, if you wanted to add Bluetooth functionality to your '03 Boxster (or other newer Porsches) and didn't want to go aftermarket, here is a great option for you.
Edit: Added a big picture of the view from the driver seat. Sorry if it is too big.