Sorry - can't help much. But it is a nice problem to have, if you understand me.
It sounds as you say, that you are almost there. I would start by seeing if you can trace where the head unit is expecting the audio to come from - either separate RCAs or similar, or on the same connector as the CD changer control. Also with everything powered off, check to see if there is a ground connection from the RCAs that you have added to the Phatbox to the power ground on the Phatbox. I would not be surprised if the grounds were separate. In that case you would need to make sure that the ground on the RCAs does somehow get back to the head-unit ground. But be careful - you could easily end up with a load of alternator or injector noise on top of the audio.
You could just try temporarily joining the audio ground on the RCAs to the Phatbox power ground, but have the volume set low - I would expect significant interference if you did this. On my Volvo install (see the web link under my user ID), the audio comes out of the 26-way connector, and I had to put in a ground isolating transformer (not expensive) in order to clean up the audio - it works very well.
Sorry you couldn't find much on the Porsche set up - but I hope that you get there. It will definitely be worth it.
