Hi, my name is Murphy and I'm a Himalayan cat. The lady I own(frndlyghost is her username) says you might try putting wasabi paste on your car cover. You can get it in the ethnic food section of your grocery store. It might look a little weird, but it will keep the cat away, and it's very humane.
Being a cat myself, I think the stray just wants to drive your car. Why not give him or her a set of keys? Oh wait, cats can't drive. No thumbs, and our legs are too short to reach the pedals. Guess you should just try the wasabi.