get ahold of XM...I am sure you can add their car installation kit to your car, and get XM.
go to their website, and click on ACCESSORIES. It's a flash-type website, so I cant link the part, but click and find the car accessories...they sell an antenna for $29.99
You'll have to ask the stereo gurus how that hooks to your factory radio.
Most local stereo shops would know because they do this sort of thing for a living. Not that they have to install it for you...but they should know if all you need is a XM antenna to get your XM-ready factory head unit to receive a signal.