Glad you asked...
The best way to keep mice out of cars is create an impassible barrier around the car on the ground. This is not as hard as it sounds. You can buy some aluminum flashing, which is just sheet metal. Build the barrier in three sections so it is easy to get car in and out. It is called a car corral. You need only have a wood base to hold the flashing standing up. Make sure mice can not get under it, so it have on stone, asphalt or cement. The barrier should be about 6 to 12 inches high. You can find more information here: http://www.hastingsreserve.org/mammals/M...
Then again, this answer might help you...
One thing you can do to try to keep mice out of your car is place a couple of unwrapped bars of soap on the floor board under the seats. Mice do not like the smell of soap and it will act as a repellent.

