Are you a software guy?  Check out the Kurt Grandis video below.  Kurt built a Robotic Squirt Gun to keep Squirrels out of his Bird Feeder.  Its a way cool project.  I think that would work even better for cats on the car.  I'm wondering if you can squirt the cats before they get to the car, but that depends on where its parked, etc... I guess you could also use a large puff of air instead of water.  I think that would do the trick, if your neighbors could stand the weird noises at 2AM... 

[Video here.][2] 


  [1]: http://boingboing.net/2012/03/26/howto-build-a-robotic-squirrel.html
  [2]: https://www.youtube.com/watch?v=QPgqfnKG_T4