How to keep mice from getting into your car?

[Squirlz]

But then your car will smell if mothballs and that smell is hard to get out. *speaking from experience putting mothballs in the attic to get rid of mice - the smell permeated into the living quarters*

I go to a lot of car shows and so many uninformed people use moth balls in their collector cars. They reek and I steer a wide berth around them. Undercoating will do nothing either.