A peltier seems way, way too complicated just to cool down the phone in your car.
Why not get a vent mount for the phone and just have the air blowing on it all the time? Even if it's just the fan and no A/C, that should dissipate some heat through airflow. You could also just get a portable fan like this and have it blowing on the phone:
http://salestores.com/ko4012vmicar.html
Out of curiosity, what do you use the WiFi for on the road, anyway? Wouldn't you have to switch networks like crazy?