I've noticed this before and still don't have a picture, but...
Today it was lightly raining in the morning. As always, I turned on the rear defogger. I do this because I have noticed it clears the water from the rear window.
It takes maybe a minute to start, and then you can see the water being repelled from the areas where the resistor stripes are, and eventually, the entire area covered by the stripes is clear. The area above and below remains covered in water.
If you turn it off again, the water fills back in and the rear window is covered again, more rapidly, perhaps a minute.
I can't imagine why this might happen. Any ideas?
UPDATE: yes, it's clearing water on the outside of the window, not frost on the inside. It does that quite well too, but that one I completely understand.