[f5fstop]

They do a great job of hiding the Onstar fuse...
From the battery positive cable, you will see three red wires. One goes to the underhood fuse block, one goes to the starter, one goes to the Onstar and XM module. In that one, you will see an inline fuse. I believe it is wrapped up in tape, but it is obvious and easy to locate.
Not saying this will help, but there was a either a bulletin or preliminary bulletin on this problem. You could just disconnect the battery positive, but this resets all the saved functions, and better just to pull the fuse.