I have been plagued with the same mice problem for years as my truck has been stored since '02. Mice were living in my headliner, behind the dash, under the seat and in the cab corners between the outer skin and inner skin where the only way to access that is the fuel tank filler neck hole in the cab. I used a shop vac to suck out the 8 baby mice from the cab corners, found a vent hole? in the bottom of the cab just above the passenger fram rail and plugged that with a closed grommet. I then plugged all of the holes in the firewall (speedo cable and wiring harness) with aluminum foil.

I found 20+ mice in the cab before those fixes and even found a skeleton in the front cab corner below the door hinges after drilling an access hole.

A trick the farmers use for their combines:
When storing a vehicle for a few months wrap aluminum foil around the base of the tires. Mice hate foil and this prevents them from climbing your tires and getting into your cab / engine bay.

 

Another item to consider - stuff steel wool into access holes (not chewable) and I'm told it hurts the feet of rodents. I've used moth ***** or moth flakes to also keep cats out of the flower beds.