Rodents are pretty sneaky and can get through some very small spots. I am not sure what you can do to the car itself to prevent this. You may wish to simple put out some rat poison near where you park. Years ago, when I lived in some rural areas, I had mice create nests in some really small areas (actually got in an air box and created a nest that plugged the air filter), and wondered how the heck it happened, but in the end, there is nothing you can really do except try to keep them away from the car with some poison or something. Just be careful if you have any pets......