I know they were talks about a merger. but i can't wait for that. xm is'nt as good as sirius. i don't like that intrusive antenna that is on there now. both the aftermarket sirius and xm antenna are twice as small as ours.
Onstar does use that antenna. The XM receiver is located under the glove compartment. Just remove the green plastic piece off the end of the antenna cable. It will slide off when you pull out the white tab on the side. Then it will plug into any Siruis reciever.
Your Onstar Satellite Link and Cell Phone Antennas are run through the same antenna...but you are in luck because a lot of people are using the factory antenna after the switch with no issues. You can even go aftermarket with the headunit and use aftermarket XM tuners and still use the factory antenna...
I dont think its the service that matters, its the manufacturer of the stereo.. I ran into the same problem in my Dodge. I had a Sony XM player and I switched over to an Alpine and had to buy all new shit. Im sure they may be a company though that makes an interface cable to use the stock antenna