A recent course I took suggested ordering the driver out of their vehicle, with license and registration in hand, while you maintain a position of cover. I was, of course, very skeptical of this until we did some Simmunition training on vehicle stops.
If a person is intent on shooting you as you approach their vehicle, YOU are at the disadvantage. If a person is intent on shooting you and you order them out of the vehicle, having them keep their hands in plain view while you are behind cover gains you some advantage. Of course this assumes they get out of the car. IF they do get out and start shooting, you are in much better position to return fire.