Dude, I'm with you. I have frikin 7-8 cats frequent my backyard every day. I'm an animal lover so it wasn't that bad. UNTIL I noticed that recently they like to climb up on my car and I've even seen 2 laying on my trunk. It's annoying. They dirty the car, scratch it with their nails, and etc. I have to always keep a car cover on which is so much work.
My friend told me about a device that emits a high pitch frequency that annoys cats, dogs, rabbits, deer, etc. I might look into that. It's called "Yard Guard".