Actually, I'm not absolutely sure but I think the nav unit is "plug-and-play." I heard this in an earlier discussion about it. Supposedly a guy ordered it, installed himself and it worked.
It can absolutely be retrofitted but may cost upwards of 5000usd after parts and labor. (this number is for cars with the PCM screen already, so adding the module for cars with the nav display already may be cheaper).
You can always get one of the aftermarket units that are out there, the top of the range garmins are AMAZING and also portable from car to car.
On a GT3 the cost will depend on whether you have PCM or not. If you have PCM then adding Nav should be about $2.5k. If you don't have PCM it will cost a lot more...if possible at all.
I had navigation installed in my 2006 Carrera S a month after purchasing the car. It caused a PCM software problem after installation, causing my PCM to lose its mp3 reading/playing capability.
They ended up ordering a whole new PCM and after logging in enough problems, Porsche eventually released a software upgrade to fix this (I forgot the campaign number).
I agree with the comments above about aftermarket navigation. Although I prefer the looks of the stock Porsche navigation integrated with PCM, it really is NOT user friendly.
Looking back, I would buy an aftermarket unit, since the stock system is such a pain to use and because I don't use navigation that often.
My dealer strongly engouraged me against an aftermarket/retrofit install of the Porsche Navigation system. The primary reason was the lack of quality map support in some locations. Instead I purchased a Garmin and love it. Now all the cars in the family have Garmins and I would not think twice about doing the same in the future.
The Garmins have excellent and accurate map support in NA and in Europe, have extensive Points of Interest, are easy to use and integrate to your cell phone (blue tooth).
My favorite Garmin currently is the Nuvi 660. In the 911 I use the Nuvi 360 because of the size.
If I was ordering a new Porsche, I would spec the Porsche Navigation System. But with the Garmin in the car, there is little reason to go with the retrofit.