Ultra-sonic zapper - they'll soon get the idea if you're able to zap them as soon as they approach the car.

Trouble with these things though is the animals seem to know as soon as you've got one and they don't bother you any more - I was sooooo looking forward to using mine.

I believe you can get special electronic devices that will detect cats nearby and emit a high-pitched sound, inaudible by humans, but will drive cats away. Maybe you could actually fit one of these to your car, under the bonnet [hood]? Then, wherever you park your car, it will be safe from these feline pests. It would be cleaner than tiger dung or fish paste, and harmless to the defenceless creatures (well, defenceless apart from a lot of sharp fangs and a set of vicious claws, that is).