I'm with you here. We have the vette under a carport with a cover but the feral cats in the neighborhood climb up the cover at the back (the cover is getting shredded) and then sleep on top of the car.
We find fur and sand but no pee yet. Haven't seen scratches either but they may be there. We have some of those big water pistols we shoot at them with if we see them around the property. The dogs go crazy when they see them, but the cats are smart. They know the dogs are in the fence and they are safe just on the other side or on top of the 6 foot wooden part. It really pisses me off though. Some people left their cats behind when they moved and now there is a population of feral cats that no one owns. Animal control can only do so much and we can't kill them, it's not their fault they're stray. One good point though is the lack of rats the last few years
Guess it's an ok trade-off. Hope you can get the smell out of your vette