Although these are very basic photographs, it shows the importance of angle. Simply by positioning the person and the camera a cetain way it brings out sections of the body larger and these two photos work very well with an unusual and unrealistic element.
Resting the camera on the dashboard of the car to keep it relatively stable would be a good approach. Then it's just the car's movement and the cars outside that will create the blur and not camera shake. This will have to be experimental as far as shutter speeds and exposure are concerned. Obviously night time would work best if I am going to record light streaks. The camera pointing at back seat and kids with blur outside?