There is a product sold in the upper Midwest called "Cab Fresh." It is sold mostly at farm and feed stores - you can get it at TSC up here (a national chain). The product was originally intended to be put in combines and tractors that are at rest for a good portion of the year.
One box (about $15.00) includes 4 pouches of mothball/cedar chip (sort of) stuff. You hang it in your truck and change it every three months. It has been working pretty well for us.
I've got it in all four trucks and my Honda - the mice had even found their way into the trunk! It's pretty strong smelling, but it works.
