I had this same problem with my GTO. Since it sits under a carport a lot, I have had it covered since the cat problem started.
First I got the cover and tied it down on all four wheels as well as the middle. Then got a rodent/animal repelant mounted next to my car.
Next I bought 3 cages and sent close to a dozen cats to the humaine society. Finally I caught a little raccoon. Started leaving cat food and leftover vegetables near my car. Every morning I'd get home from work (left my car under the carport) and there would be a raccoon in one or two of the cages. Now no more cats. I feed the raccoons, they crap all around the carport and either kill or scare away all the cats.
The best thing is, the raccoons don't get on my car, don't spray my tires, or scratch my paint.
Feed your raccoons and let them crap all over your yard.