I keep my SRT6 roadster in an "Airchamber" Air Chamber Car Storage Car Capsules Auto Car Cover Protection. The "Airchamber" is in a 24'x 26' Clearspan fabric garage. The garage is set up with flourescent lighting, 75k btuh propane heater, ceiling fan, and wood floor. Having said all that, it still is virtually a "tent". Mice can come and go easily.
The Airchamber has 2 fans which pull in filtered air and keep it under positive pressure. There is always airflow which keeps the car from ever getting condensation on the surfaces. The theory behind how it keeps mice out is this. Although there is room enough for mice to get in at the bottom of the zippered panels, there is also air escaping at those same points. The mice are not looking for a drafty place to nest and don't enter.
My car has been in there for almost 3 years with no problems...knock on wood. My VW camper is also stored in the same garage and not in an Airchamber enclosure. For that vehicle, I use Soma, Mouse Away, which is a spearmint/peppermint oils concentrate. It comes in a bottle with eyedropper. You put drops on cotton ***** and place them in the trunk area, glove box etc.. Also you can place drops directly into airvents and recesses where you'd think mice are likely to travel or set up house. The theory behind this stuff is it messes with their eye and nasal passages, as too much Vicks Vaporub does to us. Mice rely on their sense of smell for survival. Also on the plus side, your car smells really nice..Who doesn't like spearmint and peppermint.
I still keep snap traps with peanut butter scattered along the perimeter as a 1st line of defense and as an indicator of mouse traffic.
It is a constant battle. Check the traps as often as possible...
That's my .02 cents.