Seriously, it may be that the hood of your car is simply warmer, than the surrounding ground, and cats like to perch and snooze on high ground.
So you could set up something nearby, that is both warm and of a similar height, that affords easy access, for cats.
As for trying to reason with the kitty's caretakers, who leave it out all night in the relative "coolness", you will likely have better luck herding cats. So a heat pad on the lowest setting under a folder beach towel, set nearby at a similar height, may give you a paw print free car and an additional nice feeling inside. Cats are lovely sweet creature IMO, and they deserve a warm bed.