Is the Sirius radio connected directly to the audio (or AUX) IN of the radio, or is it communicating with the radio via the FM transmitter? If it's communicating via the FM transmitter, they go bad and get even weaker than they are. When you get near large cities, where there are lots of FM radio stations, the bleed-over from nearby stations can be power enough to kill the FM transmitter on your Sirius unit (without you actually picking up those stations though the radio).
Also, if your excess antenna wire is coiled into a rather tight loop, it greatly reduces the signal strength from the antenna. It should be loosely coiled, no tighter than about 10 inches across.
The antenna really should be on top of the vehicle, although radios on the XM network are more picky about that than the Sirius network radios are. The satellites are more southern than they are directly overhead, so anything metal, like the roof of a van, in between the antenna and the satellite will result in signal degradation or loss. Big truck side mirror mounts usually works well with big trucks, because many of those cabs are fiberglass rather than metal. The satellite signal will travel through fiberglass and insulation pretty easily, it's metal features like roof racks or aluminum framing that block the signal. But I think the problem is more likely one or both of the two problems mentioned above rather than satellite antenna placement.
