I just purchased a 2007 Grand Prix GXP. It has a stock Monsoon CD Player w/XM. Car also has Onstar but I'm not subscribed and no plans to.
I'm going to install a Pioneer Avic-Z2 Nav head unit with all the options. I have all the info I need for using the stock XM antenna so I don't have to install another XM antenna for the after market unit.
My question is, can I use the stock GPS antenna from the onstar unit for my Pioneer Avic-Z2?
I have read a few forums that say I can and a few that say I can't. Does anyone have a definitive answer.

Note this: I had a Circuit City installer tell me he couldn't install any after market HU while still using the stock speakers & Monsoon amp. He said it would burn up the Monsoon amp. He said I would have to install all new speakers and ditch the amp. He also said I would never get an after market HU to sound as good as the stock system. I don't know if he was that stupid or if he thought I was that stupid. Needless to say, I walked away and will never go back. Also, every installer I talked to said I couldn't use the stock XM antenna and wanted me to purchase one. I have since learned otherwise.