That was my biggest worry when I put my Dodge away for the winter last fall. Because it has an excellent original interior that smells really good, I didn't want to put any dryer sheets inside the car. I had the car up on jackstands and scattered them liberally all over the garage floor underneath it. I also stuffed some in the engine compartment, inside the coil springs where there might be a route for them to travel and up where the pedals go through the floor into the cabin. I threw a bunch of fresh ones around about every four or five weeks when the smell seemed to be subsiding. I also had six or seven small deli containers with a half dozen mothballs in each strategically placed around, including at the base of each jackstand. Plugged the exhaust, road draft tube and drain holes in the spare tire well with steel wool and left myself a note the I had done so taped to the car cover! LOL! I also had regular cat patrols through the garage. I was greatly relieved this Spring when I discovered that the old bus had escaped with out any damage.
I have been told by a veterinarian that a mouse can squeeze through an opening only 1/4 inch high. They can do an incredible amount of damage, not just by gnawing on stuff, but by peeing all over the place. A friend who works in the service department of a local Toyota dealership told me this horror story about a Prius that they got into the shop. It seems that Toyota uses a soy based material for a lot of the insulation on wiring in these cars. A customer who owned one was out of the country for an extended period of time, and took no precautions when she parked her car in the garage. The soy insulation was just too tempting for the little varmints, so they moved in and told a bunch of their closest friends about it. They set up house keeping in the headliner of the car and spent the winter eating the wiring, pissing all over the headliner and making more mice. Not only was the car rendered undriveable by the damage to the electrics, but when the weather finally warmed up it stunk to high heaven! So not only did they have to do extensive electrical work on the car, but they had to replace to whole headliner!