I put my GPS antenna under the dash behind where the air vents are (the factory GPS screen location) and it works fine. I guess you don't need an open view to the sky for it to work. Not sure about Sirius, but I get full bars with my GPS antenna hidden in the dash.
I did have it on top of the dash, but I figured if it gets a signal inside the garage, why not try it in the dash... works just fine inside the garage in the dash.