Most GPS systems require a direct open path to at least 3 of the satellites....such is why GPS doesnt work inside. However, there are some routines where the GPS is enhanced by the cell phone signal strength...to make the GPS work inside. You want to confuse the GPS but not screw up the phone signal. One way is to shield the GPS antenna but not shield the phone antenna....I'm not sure if your phone has both antennas in the same package OR if the GPS antenna is somewhere else in the phone case.

Try this...hold your GPS phone up against the headliner of the car (a metal roof). Do you have a GPS signal strength indicator? IF so, does holding the phone in the shielded location decrease the signal strength? If you have a signal strength meter for the GPS signal, do some experimentation...you might find a way to confuse it.