This is a very common issue, not just on IH vehicles but any motor vehicle which has a cabin fresh air intake for the ventilation system located at the base of the windshield.
And that is major freeway for mice/rats/snakes/small children to enter the heater or hvac module and wreak havoc over the years.
In many cases, the oem solution is/was a fairly coarse mesh screening material (either nylon, polyester, or stainless) that keeps stuff out but allows air passage unimpeded. The mesh size must be sufficient however to not restrict airflow when the blower is running.
One way to retain the screen material once it's cut to size (with the cowl panel removed of course), is to carefully retain it in place from the back side using a commercial grade "hot melt" glue gun/media to stick it in place, that way you can see nothing from the outside other than the screen.
Also, while the cowl is off, you can completely clean the inside of the plenum area and treat with body seam sealer as needed to stop all the water leaks behind the dash!
