Does anyone here have any experience with their XM radio? I just decided to mess with it and called the XM company, they gave me 3 months free without having to give any credit card information. So far it is pretty good, the sound output is sometimes lower then regular radio. When I bought my car it was missing the top black antenna piece, it looks like it was slide off. I don't know if this is the receiver for the XM radio or the OnStar or both? Anyways what have your experiences been?