Car Windscreens not allowing transmission of GPS signals
There have been a few reports of the GPS not being able to get a
fix inside the car. This appears to be with multiple brands and
types of GPS (a smart phone with in-built GPS is an easy test). The
fix might be intermittent or it takes a while to lock, or it might
get a lock to start with but then lose the lock later on.
As previously noted several, GPS units (VMS and others) have been used in these cars with similar results, however all GPS units tested ok when they were sent back to their relevant manufacturers. At this point it appears to be something else that is affecting the GPS signal - in some cars there is a fine metal layer in the windscreen, that, to some extent, blocks the GPS signals from entering (this layer is there to strengthen and heat the windscreen).
If your car has this type of windscreen, the only option here is to purchase a re-radiating antenna.