I'm not too up on this, but check this out:
http://www.amazon.com/gp/product/B00...Fencoding=UTF8
From what I gather, your XM subscription would be on that SIM like card, and you could somehow plug that into a XM ready car deck.
I have the Delphi MyFi receiver, and it has a builtin antenna, but it also came with a car antenna that I mounted easily enough with the help of a friend, and a portable antenna.
xm411.com
xmfan.com
Two good websites with loads of info. I don't mean to be all pro-XM, but I don't know much at all about Sirius.