I did this this weekend. My cowl panel was in bad shape, so I pulled it and sanded it along with the wiper arms. After sanding, I hit them with gloss black Rustoleum for a few coats.
When it came time to put it back together, I put about a quarter sized glob of "Goop" on that plastic nut and the surrounding area. I also tried my best to stuff some in to threaded area of the nut as well. After that, I covered the area with aluminum tape and poked a whole where the screw comes through the cowl in to the plastic nut. I hope it works!
I also pulled back the passenger side carpet to find it along with the padding and sound deadening and the firewall "blanket" to be soaked. I towelled up what I could and left the door open some while I could. I'm hopeful it will dry up well so I can lay the carpet back down.