I used Euromotorspeed double Din installation kit. I e-mailed them the type of unit I was installing and the installed Bose system and they provided the correct harness. Just plug & Play. They also supply the double din mounting chassis and the square and curved fascia plates. Check them out on Ebay (no affiliation).
There are also a few clips on YouTube. Search Boxster or 996 Double din install.
I installed a Kenwood DNX-5160 double-din head unit with Garmin Nav, Satellite radio & iPod from Crutchfield. Bluetooth and programmed startup and main screens.
Good luck!
Some pics:
Before:
During:
Afer:
