I spoke with a SOUNDGATE, (they were referred to me when I asked Pioneer about an adapter). The man there told me that GM has a 'special' Pioneer receiver made just for them that is not the same quality as a real Pioneer. He also said that simply removing the factory head unit will not only disable the chimes, but other stuff. He said the 2 ways to do it are to bury the factory radio somewhere with all connections except speaker, or to wait for the special adapter to be made.

Scosche is working on the GM LAN adapter right now. As soon as they have it completed I will post info on the forum.

6 1/2" is the direct replacement size:
Attachment 45034
Attachment 45035
Those are the factory speaker rings that the Audiobahns are attached to. ;)