Cats hate sticky things that will touch their feet and they hate smelly things. The smell issue is different with every cat so you may have to try different things. Tea tree oil, vinegar, anything strong smelling could work. Soak a piece of fabric with it and put it on the car where the cat likes to sit. Maybe just keeping a wet piece of fabric on there could work. Cats also hate unsable surfaces, a pieces of slippery fabirc that slides off the car when they try to get up there could work. They sell electrified mats for $50 or so that are battery powered and shock the cat when they get on the mat. The mat is clear and flat so the cat can’t really tell it’s there. I tried that with my own cats. It worked. It doesn’t hurt, I tried it, it just scraes them because it feels weird. They also sell pest away sprinklers that are motion sensitive. Gaiam.com has those.