red pepper works wonders...cheapest and most effective anti-cat agent that i found. Humane too. Gotta be humane.
Besides running the animals off harmlessly, its kind of entertaining waiting and watching for the offending cat to tip toe into the forbidden area and get a snoot full of red pepper !
I had issues with cats using a section of dirt right next to my front door as their private litter box. Piles of cat crap that stunk constantly. Not wanting to shoot the things, for fear of arrest or damage to property, i elected to use the brain they don;t have....
2 bottles of red pepper sprinkled in the area did a great job,. And was fun to watch too !
Perhaps if it was sprinkled all around the car each evening for awhile and a little on the top cover that would end the problem. Just be careful not to get a dose yourself...it causes instant sneezing and some eye, throat irritation if the pepper dust is inhaled.