That's some nasty stuff. I wonder if you could power blast it with high pressure water to get most of it out?

A regular garden hose washes it out pretty quick.

There is another article related to this one, about re-sealing the cowl
https://www.fordtruckzone.com/threads/re-sealing-the-cowl-on-a-80-96-f-series.657653/

On my 1996 F250 the fastest, easiest way to unclog those cowl duck bill rubber drains is to crawl under the truck and literally grab them with your hand. No tools required.

Once you've found the right place, it's pretty easy to just reach up and around and grab the duck bills and squeeze/dig out the packed crud with your fingers.

Mine were clogged solid with mud and organic material that had basically turned to adobe brick over the years, so high-pressure water didn't work. And my plastic inner fenders were unfortunately far too stiff and brittle from years in California to use the suggested method in the above thread.

Pro tip: wear goggles or safety glasses as that crud will fall and bounce everywhere.

You can flush with a regular garden hose a couple of times in between digging them out with your hand, to make sure you've got every speck of dirt out.

The 1996 F250 drains are not difficult to reach from underneath the truck (the driver's side does require a bit more contorting and finding the correct angle to get in there). I can't speak for other years, but I'd love to hear back from the 80-95 people to see if it works for you.

Thanks, Man! Exactly what has been the source of my problems!