Like it would be illegal, unless there was no way to see you. Like if the windows were clear, anyone could see his butt pumping away. Like in a limo with tinted glass, this happens all the time. Like?
If someone can see it, then it's lewd behavior in a public place. In other words, if a cop or someone else sees you, you're in trouble. Otherwise, it's just kind of weird that you would want to do that with someone else in the car (the driver).