I don't have a Mustang but I do have one of its brethren. It's a 1984 Merc Capri RS 5.0 that I only drive in the summer, when it's not raining, and I store it in my garage in the winter. A couple of years ago I had a Red Squirrel in the garage that had chewed its way through the cedar siding, sheathing, insulation and pegboard. I patched everything up and the squirrel was gone. In the spring I pulled the Capri out of the garage, lifted the hood and discovered a squirrel nest next to the firewall - complete with dead babies. It was nasty to clean out. Now I store the car with the hood open so I can check on any critter activity. Also, if any one of my cars has been sitting for a week or so, I'll open the hood and check for mouse nests and, yes, I've found some. I live in a very wooded area so this friendly tip may not apply to everyone.
Dick R.