As long as there isnt metal in its "view", it will be fine.
Do you have a metallically tinted windshield? In my Honda, the windows all have metallic tint on them (4 sides and front back) so signals inside are crap. A good way to test it is to slap it out the outside of your car (its magnetic so it wont be hard or damaging) and see if reception improves.
But the 4-5 minutes you talk about is after it is warmed up right? How many satellites are you seeing and are they at a good strength?