Live traps for mice work well if you check them frequently. In the warmer months, I use 5 gallon buckets and pour about 1/4" of used fry oil in, this intices the vermin in, but they get all slickery and can't jump out. Not a good idea to put the bucket in your car, it'll smell the interior up like old french fries. They do need a way to get to the top of the bucket however. Set it next to a low shelf or stack of stuff. Poison works pretty good to until one dies in the car and reeks it up. Usually they eat the poison, get thirsty and leave for a drink to never return, But...
I keep my cars covered with old (soft) blankets sewn together to make one big blanket then cover the blanket with a sheet of plastic. Just enough to come down to the sides. I roll the windows down about 6 inches to let it breath. It can still mold up, but not near as bad if it where closed up tight. The plastic keeps any moisture from dripping on the car and keeps the dust out of the open windows, it will also deter cats from lounging on it.
If the concrete sweats or water washes over it, I have heard of people parking the car on a sheet of plastic to keep the moisture from wafting upwards and rusting the under carriage, even jacking it up on jack stands will help and using a fan to keep the moisture moving.
These are the best I've been able to do, but they lack quite a bit. The ideal option is to store in a cool dry place, but you already know that.
I haven't tried this yet, but I heard that if you put a couple of bars of Irish Spring soap in open plastic containers inside the car, mice will stay out because they don't like the smell. Plus it will keep your car smelling good.
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