[jmudler]

Here is what I did. I purchased METRA 99-2011 install kit for the aftermarket double DIN head unit I will be installing.

I used the pocket in the kit to replace the onstar unit. I removed the black "front face" of the onstar unit. This provided the mounting bracket for the pocket. I cut a hole in the onstar face plate to allow the pocket to slide in from the front and hot glued it in. The onstar face plate is curved so I sanded it until the pocket sits flush/flat. About a 1/4 of a inch of the onstar face plate is visible on the sides of the pocket, so I use finer grit sandpaper to smooth out the sanding marks.