The satellite radio in the car is only picking up on land based antennas when they are avalible, as they don't need line of sight to work. I have driven through the sticks of NC and VA where there is no cell phone service for miles, but the XM never lost its signal or faded out. If theres no cell towers, you can be sure there are no XM broadcasts either.
As for the XM antenna on my car, I have it routed through the sunroof opening, though its probably easier in retrospect to have it mounted the way it is on most cars at the back window. If you peel up the weather stripping around the window there is actually a channel there, the wired go inside the channel, wrap around the window and then enter into the trunk through the seal. I drilled a hole in the sunroof opening, though removing the headliner would have been a better way to do it as well. You can also just put the antenna on the back deck of the car, but you will lose some reception quality. I know of many people that have it like this also and they don't seem to notice much signal loss, they mostly drive in very populated areas too though.
I love XM, and recommend it to everyone. I like Pioneer and Alpine which is why I have XM, and it has more Rock stations then Sirius and many stations are commercial free now also. When driving to NC from NJ, I never put in a CD anymore, just have XM on.