I just installed a Pioneer AVH-2440NEX, Metra 99-9604b dash kit and Scosche LPPE15 wiring interface from Crutchfield in my 2005 Cayenne. Absolutely terrific sound. Existing amplifier and subwoofer work great.
I highly recommend using their $25 ReadyHarness Service tp pre-wire everything. Like others in this forum, I needed to trim the dash plate to get a nice flush mount.
The USB AUX adapter from Amazon fits perfectly in an unused dash hole.
Total installation was was less than $700. $350 for the radio and about the same for the harnesses. The Axxess ASWC