neither did I, but typically if you have Onstar or XM they will be seperate plugs, and shouldn't get in the way of installation. They will usually have a harness to adapt to the Onstar system as well, as soon as they come out with it!!
I don't know how they would get it in any other way.
They probably would just cut the wiring though. I just didn't trust them to get the big boomerang piece off without any damage. I knew I could take my time and back off if something wasn't going smooth.
It took me from about 10:00 in the morning till about 4 or 5. Would take a lot less now.