i know that hunters have a motion activated camera that takes a photo when anything gets within so many feet (its adjustable)
the only problem is it has to be pointed AT the subject. so this means it has to be outside the car.
i am not sure if it would work from inside the car, with the flash and all reflecting off the glass.
have you considered keeping a log of the nights this happens, and on a suspect night sending it outside in an area within view of the car, but sort of hidden and kept watch yourself? (with a flash camera)
also have you discussed this with the police?
do you have a restraining order out on this individual?