Interesting. We too have a cat problem and have tried most things.
We've had two ultrasonic things; it might be my garden or just the make, but both were useless. Even caught a cat asleep next to it once. Perhaps it was deaf, although it seemed to respond to "you little **** get your **** out of **** or it will be *** on your **** paws" well enough.
The one thing that DOES work (for us) is Silent Roar, which is indeed dried lion dung. Really does keep the cats off, but it's a tenner a box, which doesn't go far, and you have to renew it every so often.
Hadn't thought of putting it on the car; I should as we do get pussy pawprints on the bonnet in the mornings sometimes.
The ABSOLUTE guaranteed device, if anyone cares, is called a Water Scarecow. It has a PIR, and you attach your hose to it. When it detects movemetn s is squirts water and makes a loud rattle. I can promise that will keep the cats off, but you have to have £60 to buy one, leave your hose connected to it permanently and of course aim it at the right place so it doesn't soak you everytime you go into thegarden (been there done that).