Built-in car satnav systems only use GPS as a sanity check, they mainly use the inputs from the road wheels and electronic compass.
My old car had persistent problems with water getting into the antenna assembly and rusting things up, so I was completely without GPS signal on several occasions. Did it make any difference? None at all if you were in a city because every time you turn the system is able to recalibrate your position.
Biting the hand that feeds IT
