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General placement of antennas:
Sirius, on top of the vehicle. Sirius has three satellites that move latitudinally across the US in relays, thus one is over the US at all times. Therefore, placement of the antenna on the roof will allow it to have a clear shot at the satellite at all times, no matter what direction you are heading.
GPS, has multiple satellites (26 I believe) that run longitudinally across the US. So the antenna can be placed under the plastic in the dash, as close to the window as possible (factory installation on most GPS antennas). Since there are multiple satellites in all directions you travel, you should be able to obtain at least 4 satellites at all times. Three satellites are required for specific location; four satellites give 3D data for units that specify altitude.