GPS is a very faint signal, and needs to be pretty much line of sight with the sky.

It will go through thin ( 1/8" ) plastic or glass, but sheet metal of any kind is a formitable barrier.

I've been suspicious of that little plastic square ( or rectangle, not looking at it now ) in the center of the dash, near the windshield. I can't confirm at this point, but other than the outside of the car, this would be the logical place. ( also close to the back of the Nav unit itself )

IT IS POSSIBLE ( but improbable ) for the GPS signal to be integtrated into the AM/FM whip antenna on the outside of the car.