Advanced Auto Design put an iPad in a 2003 Range Rover. It is integrated via Bluetooth so all you have to do is put it into the mount and everything is streamed through your car’s stereo. You can also use 3G to have internet anywhere you are. Currently the 3G iPad is not available but you can use an aircard as a WiFi hotspot or even get 4G with Sprint’s card.
I am sure any custom car shop can do this for you, but I am sure it wont be cheap. If you want a real GPS solution you will need to purchase the 3G iPad as the WiFi only model does not have a GPS module built in.