Turned on the vents and shredded Kleenex came out. Lifted the rear seat and found a mouse staring at me. I suspect the little bastard got in through the (formerly) rubber part of the transfer case shifter boot, then ran up into the dash through the floor vents. Long story short, there's one less mouse in the world, the truck got a good vacuum, and I made a surprisingly decent-looking shifter boot from part of an air mattress. (Probably would've looked better if I hadn't used the part of the mattress with the inflation instructions printed on it.) Hopefully that keeps the rest of them out.