One of the best sexual encounters I had was in a car with a moonroof... during a thunderstorm. The light display and the rain was amazing, as was the sex.
My old car wasn't that big, either - it was a Nissan 240SX... and we're both tall, too. I can say from experience that car sex is nice, but it's much nicer to not HAVE to have sex in the car.
If you MUST, however, front seat laid back with her on top or passenger seat laid back with him kneeling on the floorboard and her spread and reclined on the seat are probably the two best positions. In fact, there was a stain on my seat until the day my car was traded in from the latter position - she enjoyed that one a lot!