On your roof, passenger side, is the Sirius satellite (only) antenna. Sirius reception also uses a terrestrial signal that is received thru your AM/FM antenna, located in the passenger side rear quarter window. Sirius has a vast network of repeater antennas in the US that allow for reception when there isn't a clear view of the southern sky. Work-wise, I had a part in this back to the mid 1990's. Sirius output to your head unit is by ways of a tuner, usually a small metal box located somewhere presumably under the dash/behind glove box area.
The GPS antenna is tucked away under the dashboard, literally. Its not exterior mounted like you might expect. The red slash means its not receiving "ideal" GPS signal from multiple satellites. There are 24 satellites in orbit, normally you have access to a minimum of 4 for triangulation and comparison. Good reception in the US is receiving from 6 to as many as 8. Less than 4 sources reliably will give you this red slash, which simply means its insufficient but is still working. Usually I see this when driving along heavily tree-lined streets / under a thick tree canopy or in valleys. These conditions will also interfere with Sirius terrestrial and satellite reception. Heavy downpours give the same results.
Having metallic window tint also hinders GPS reception. If you've added metallic tint to the rear quarter windows, you can be hindering Sirius as well. Only the installer would know if its metallic tint, or the more desirable carbon, dyed, or ceramic variety.
When you see this GPS icon and your Sirius starts cutting in and out, simply compare active reception with your cell phone or FM channels. If they are all degraded, its environmental conditions.
If your cell and FM are perfect, trouble could lie in your head unit. Which would be surprising somewhat, as the Sirius is via a separate tuner. If it were to go bad, GPS is directly unrelated and vice versa.
Seems like I said alot without really saying much.
