There are two problems nobody has mentioned.
1) Some car has a layer of thin metal film on the glass for UV protection. It would block radio wave. You have to install an out of car antenna for GPS.
2) There are two generation of GPS receiver. The first generation is less sensitive. It would lose signal more easily. Until last year there were still many models of GPS on sale using the first generation receiver.
Only Garmin and Tomtom One are worth buying. There are too many models are just a device runing windows mobile with GPS software.