As other's note, it's a mice problem, not a vehicle problem. IME, the bigger exposure to mice making a home out of the vehicle is that eventually it becomes an odor issue that cannot be remedied without pulling apart, replacing, and disinfecting everything.
You absolutely, positively, gotta find a way to either (1) get rid of them, or (2) keep them out of the vehicle. There's many methods that are largely up to you including cats, poisons, traps, deterrents, sealants/screens, etc, but likely a combination of these things given they are persistent buggers for food and warmth.