I dont know how true this is, but if you get a water bottle, or clear bottle, half fill it with water and leave it sitting somewhere they will see it....say on the windsreen lying on the wipers. Something about them not liking with their reflection in the water....dont know if its works or not, but its a no cost, no harm, approach.

Also, Ive heard, if you know the path the cat walks, its usually the same path every day...and you can obstruct that path in some way, they dont like it, and change to a new path. For instance, the cat walks your back wall, and you to put a sheet of timber on the wall, that blocks them, they'll change their route....you might be able to direct them away from the car.

If they dont like citrus, it would be pretty easy to leave a few traces about the car or indeed their path too the car.
Also, not sure how safe Jeyes Fluid would be for the paintwork.