I have a similar issue here. The car is in the unheated garage and mice find all sorts of stuff to make a nest, in or out of the car.
Currently I have 3 traps with peanut butter as bait. I will go for a week or more without any action and then for a couple of days I can have enough mice to almost make a coat..
I usually have the traps tied to something that weighs a few pounds as the little critters can trash about and dissapear into or under something I won't usually move till spring. You are right in that they dry out and don't usually smell. The odd one in the summer will but not winter.
I have gone away for a week and the little buggers had started to move into the trunk and spare wheel tub. Under the windshield cowling was another spot.
Don't put traps in the car, just encourages them.
Chris