StijnK:
The big question is, can we read that data that comes from the GPS module and send it through the wifi-shield?
Sure you can. You do know that GPS won't work instantly and needs minutes to boot up and won't work inside a garage or tunnels, right? Unless the car only passes heavily populated areas to pick up wifi everywhere it goes, you will only have bits of information while the car is near a coffee shop or public library. The wifi range is so short, thanks to all other wireless signals in the airway, you are better off just printing off which wifi network you are on instead of printing your GPS locations. Both probably have around 5m range/accuracy so really the GPS sensor is useless. My home wifi connection is lost once I'm about 5 meters away from my front door.