My Indy told me all you have to do is put the cats in there for a couple days for a few hours now and then. Once the mice get their scent, they immediately start looking for a new place to live. The gig is up.
Just a word of advice, though I'm certain it doesn't need mentioning. If your cats are like mine, you'll wanna cover your car. Or you end up with this:
http://www.youtube.com/watch?v=dqGIUGF3PzY