Hi Dorf,
It's not just Ford. I have a 2004 Explorer, Pontiac Van and a Dodge Ram Pickup and have had mice nesting in all three. I'm going to be contacting NTSA, hoping they can make some kind of regulation. Anyway, the only AC/heat vent that i could find was as you mentioned, in the trough under the plastic grille at the base of the windshield. It is only there for looks and anything bigger than a chipmunk.
I custom fabricated an aluminum mesh screen, cut to fit the opening and fastened with screws and posts from the grille. I had to make a very precise pattern and cut holes for posts, wiring and other obstructions. It firs over top of the wiper motor. Nothing smaller than a lady bug can get in there now.
Had to also put a mesh screen over the air intake openeing, just behind the headlight.
Rodents can cause a lot of damage, but if you have comprehensive insurance, it should cover all except a small deductible.