Bug: Have you considered this option:
Apparently they make a driver that adds to the airports abilities.
Now,I've yet to get my machines in my car, but if the problem has not been resolved by then I may give it a whirl. On the other hand, I may also just stick a relatively inexpensive router in my car to pick up wireless wherever I am, and with a few ports on the back networking to any other machine in the car is a snap (PC or MAC). In addition to that, if you ever bring another machine with NIC or wireless capability, you can instantly connect to the router hidden in your car.
That in fact might be the nicest option as you will not have speed issues over at least a 10/100 NIC connection.
Mind running another wire in your car?