I store my Porsche (and another car) in my garage with just a cover. Never had a mouse in the garage. Only thing I can think of is the fox family living under my shed on the hunt each night. Also noticed my backyard vole problem disappeared 👍🏼
Making the assumption it's under the hood and foxes, and other large critters, are not getting under the hood. You could seal it in a Tupperware bowl with a small mouse sized hole - zip tie the lid shut.
Zip tie (or other wire) some of these under the hood. They eat it and leave the area to get water and die. I use these in my attic and shed and you can see when/if you have critters by how much is eaten. I now have a fox family living on my property and have not had a mouse problem since -...