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[Bear Grills]Pee in a circle around the car. The testosterone will let the critters know that it's already someone elses and they'll back off. Just make sure you do this and not your wife. [/Bear Grills]
...and get arrested for free-willy'ing in public Urine does not work - even tried badger piss you can buy online, cats smell it and get inquisitive.
Travis... on a serious note... car covers are made to breathe to a certain extent so as to not trap in moisture. Using two should be fine for a short period of time. You can get cat repellant in a spray bottle and can get powder for the ground. Spray the areas of your car cover then make a barrier around it.
If that doesn't work well then come over and get this: