You might not think it, but keeping mice out of cars (especially those that are infrequently used) can be frustratingly difficult. They seem to enjoy doing expensive damage, nibbling on important wires and getting into the climate control system behind the dash.
The first thing to do to make your car less appealing to mice is to clean it well, especially the interior and luggage compartments, being sure to remove any obvious sources of mouse food (garbage, cardboard, paper, human or pet food, etc..). If you’re storing your car inside ensure that the building itself is also clean and in good shape, with no obvious entrances for small animals.
The most effective solution to keeping mice away might be the old standby, the cat. A cat will not only catch mice, but their smell tends to scare rodents away. To keep mice out of the engine place moth balls around the engine compartment in breathable bags or socks. Dryer sheets should keep them out of the interior and keep it smelling fresh.