Your movement ends when getting in or out of the car, not your turn.
As long as you don't move the vehicle you can use the motion sensor.
This is partially true in the sense that the detector can't be used if the vehicle has moved. But the original question was whether an agent can move to the vehicle, get inside, and use the sensor - in which case the answer is no he can't. Hunters must already be inside the vehicle at the start of their turn to use the sensor.