About a quarter of the time, after a backpack trip, a field mouse has made my car it's home.
I hate to do it, but I don't want mice tearing up my insulation, electrical wires, and anything made out of paper, or peeing on the mats, so I leave a snap trap in my trunk at all times.
Yeah, peanut butter works good. Have to clean it off and re-bait a couple times a year. Until then, when I find a mouse, I just dispose of it (very sorry Mr. Mouse, I hate to do it but you just can't live here...) and re-set trap.