Mice had a winter party in my Cutlass a couple of years ago when I was rebuilding the engine. I have to share some of the blame... I left squirrel food in the garage. I still keep finding piles of corn here and there in my garage. I've stored cars for 25 years, never had mice problems before.
I don't have the heart to use spring traps, so I bought a couple of reuseable live traps baited with nuts. Got them all! I let them go at work ten miles away. I also heard they don't like the smell of mothballs, so I put them here and there in my car wrapped in tissue. Direct contact with mothballs can harm some material and fabric.
So I caught the mice, put food away from the garage, mothballs in my car, and have had no problems since.
95 E320 Cabriolet, 131K