I have used peppermint oil; on 3 -4 cotton balls in old washed up tuna cans put few drops on each cottonball one can infront on floor, back seat floor, in the trunk and on top of engine and every few weeks add more drops the mice do not like the smell and also makes the car and shop smell better has worked for me the last several years also leave the car shut up tight look for any place they can come in plug with steal wool. I tried dryer sheets and irish spring bar soap the mice just chewed it up. my car is stored in a non headed dirt floor shed