I haven't seen the movie, but I heard Jason talking about Charlize Theron working under the hood and her car making the sound of the engine turning over not being possible without someone in the car. This brought me back to my late teens, working on my old '66 Mustang.
On older cars it was possible to just use a screwdriver to bridge the poles of the solenoid to engage the starter motor. As long as the key was turned to allow the engine to continue running, you could easily start the car from under the hood (
Now I don't know what kind of car she was working on but it's certainly not IMPOSSIBLE and probably wouldn't require a ghost.