Don't do anything that will hurt the cat, If the cat is wandering around outside, it's the owners who should get hurt. Cats are still a semi wild animal with killer insticts and they love to eat the native wild life, having grown up in a cat household I know they can be a pain but you really ought to keep them indoors, especially at night. If however your cat is inside of your locked garage with your cars and you are doing the right thing by keeping it locked up then you could always try a spray bottle of water with a teaspoon or two of vinegar in it, spray the cat with this every time it jumps onto the car, the dilution is enough that the acid wont effect your car or hurt the cat, but they hate the smell of it and soon learn, once again thats vinegar, dont go spraying your cat with some other acid ^_^ Another thing cats and dogs both don't like is bi-carb, but sprinkling that all over your car would be messy and depending on the weather could harm the paint, however it works wonders around furniture and carpet.
Listen to the guys who posted up and said to use citrus products to repel them. People who feel the need to hurt ot kill small animals, usually have serious mental issues, and they obviously dont have kids that own pets. I knocked out this clown I was getting a lift with once years ago, (whilst we were driving along) because he tried to run over someones pet cat. Its a sad, weak thing to do to go out and kill some little kids pet...
