The Supreme Court has determined that the police have the authority to -demand- your exit from a vehicle, orthogonal to the issue of "Am I free to go?" and other detainment questions, and to compel your exit from the vehicle using force if necessary (at which point you reach the awkward catch-22 of "is this classified as 'resisting arrest'?"
"I'm going to search your vehicle, ok"
If you say "ok", then you just volunteered to have your car searched.