Are they getting into the interior of the car or just the engine compartment? You can't keep them out of the engine area of the vehicle any other way than by somehow keeping them away from the vehicle. As far as the interior there are several ways they can get in no matter what but one way to help shut them out is to set your ventilation system on recirculate when you shut off the vehicle. When the system is in recirculate mode it closes the door to outside air which is an easy way for rodents to get in. Your owners manual should explain in the heater/air conditioner controls section how to select the proper mode.
Decon, mouse trap's or a hungry cat. Can't keep them from getting in a vehicle any other way!! They are worse when cooler temps. set in this time of year. Also, mice are responsible for a lot of damage to vehicle's! Can cost hundreds of dollars to repair, so get rid of them any way you can!