I've heard using scented dryer sheets will work in keeping critters away. Also, Irish Spring soap shavings in plastic cups placed in/around the car's interior helps...smells ALOT better than moth balls. Additionally, I place cups w/ the shavings on the floor at the chassis side of each tire.
Cedar balls, chips, or blocks also works. But the scent fades quickly, and they should be replaced twice per year. I've heard that the soap trick does work, but I've also heard the smell of the soap is extremely strong and bothersome in the Spring. I've actually grown to like the moth ball smell!