I don't have XM in my mustang, but I did put it in my patrol car. I stuck a little velcro to the bottom of the antena and stuck it to the rear deck. It works great. Maybe a whole 5% less signal than when it was mounted on the roof of my other vehicle.
One thing I noticed is it works better the closer to the center of the window you can get it. When I stuck it in the corner, I would lose signal anytime that side of the car was facing north.