I also looked into changing my 6 cd stereo with a factory nav head unit , I have a 2007 GXP. What I have come to find out that unless it was ordered from the factory with the navigation head unit then you can't put one in that will work with all steering wheel controls, auto volume control because the harness is different. It was an ordered option . You can have a 2.0 usb port installed or an after market head unit but all the other control options will not work. I do not have a link to post but this information came from a close friend who is a service manager at a former pontiac dealership and said the only way to do it would involve taking out the dash , replace the harness for the nav/ipod , and put in the new unit. Not very cost effective if you ask me and very time consuming if you ask me ... I hope this information helps. I am good with cars and I'm an electrician and trying to pull this off seems like a waste off time.