Answer:
Make sure the car has been switched off for a long time. Purchase a femidom from your chemist (a female condom). Put it in the exhaust pipe and go for it. If the car is still warm, the exhaust pipe will be hot and that would not be a good idea.
If you were asking about how to have sex in a car with another person (not having sex with the car) then just put the front seats forward, jump in the back and it'll come naturally. The forced closeness of you both will make it very enjoyable.
Also, not recommended whilst actually driving, but if the man is in the drivers seat and the lady is in the passenger seat, with a little adjustment to your sitting positions it is quite good if the lady gives the man oral pleasure.