Any idea how to......

stop a cat from sleeping on top of my car??? It's a PT Cruiser Convertible, and although there has been no damage, I am worried about what MIGHT happen!
It's a neighbourhood cat, and is skittish enough that you would never get close to it. Always happens at night. The car is parked under a carport of sorts. We have started putting a small tarp on top of the car, but have to weigh it down with something on the edges so it doesn't get blown off. Not so bad to do that, but gets tedious! The cat still gets up on top of the tarp.
Left the car out in front of the house last night, and we could tell that the cat had been up there, but didn't stay. I guess it didn't want to sleep on top of the frost. Oh, and then we always get little kitty mud prints on the car!!
Hope someone has some good suggestions!

Thanks so much.

Glenda