I just read my initial response and all I have to say..man, did I come off as one of those typical sanctimonious gun forum jerk guys we all hate!
Nice discussion though, and while I still stand behind the idea that prevention is best here, and the second best is to get out ASAP while the car is still not moving, I have to say that in DESPERATE circumstances I could see myself thinking about some DESPERATE Crash the Car Movie Stunt, but something like that is a pure gamble pure and simple and probably not the best option.
I think you'd have better luck by pretending to lose consciousness +/- go into "an epileptic fit", or to look terrified, clutch your chest and say "CALL 911 I'M HAVING A HEART ATTACK" and then "pass out". And then let the guy rifle through your pockets (unless you are carrying, which obviously makes the point that while carrying you have to be even more vigilant not to let some dirt bag get the drop on you), or even to reach over, or exit to come around, in order to push you out of the car. This of course should open up other avenues of escape.
Anyway this is one of those "impossible scenarios" that is best avoided, so rather than exercise one's neurons about how to defeat something like this, we ought to focus strategically on not being caught with our nickers down like that! Is anyone offering a James Bond ejector passenger seat set up?