If there's anything better than sex on the bed, it's sex in the car. Cars have a long history with both romance and power, and that makes them the perfect companion and sex location for dominant Masters. It could be something as simple as a trip in the country, and then "making out" to a nice view with a compliant young lady, like their first romance or favourite movie. Perhaps more humiliating, being in an almost public place, and the risk of getting caught. It could be something as simple as forcing his slave to perform oral sex when on a long drive, giving the Master pleasure at the same time as he is enjoying the drive. Maybe the Master's fantasies are a little darker: with some kidnap and rape play.
In the end, cars are a source of strength and power, and often a status symbol. Like any fetish, being near the car, in the car, or perhaps even on the car can give extra impact to your dreams and fantasies.