I was just looking at some interior pics. A double DIN might fit. But, I think either the 1.5 DIN or double DIN will require some work to make fit due to some sort of depth or mounting bracket issues. The On-Star system might also throw a wrench into the works if so equipped.
Might want to check some GMC message boards for more specifics.
An FM modulated CD changer might be the easiest and least expensive route.
