I've heard that too. I'm guessing no matter how many holes you patch, if a mouse wants to get in he will. I think they can enter through very small cracks or holes.

I've also heard of a "bag" you can buy for storing cars. You drive on top of a bottom piece, and then throw the cover over the car and zip it closed. That would be a real pain though if it's a daily driver.

Do you live in a very rural area??