Rainex make one and super cheap have it. I use to put it on the inside of my old hilux when the demister didn't work.. I also use the external one it makes driving in the rain good...
I'll back up turning on the a/c. Mine is always on. Rain, hail, summer, winter and I never have fogged windows. I also use that fp1 cleaner wax stuff that I bought years ago. Cleans the windows up good and helps stop the fog.
Sprinkle bicarb through your dash mat, even your in the sun put a open container with it in the bottom and let it work. Nice big flat one not a small container