Spit and rub, right?
This actually happened to me over the past weekend in my MS. After driving for about 2 hours all of a sudden the windshield started fogging up on the highway at night. I had the fun task of trying to locate the climate control icon on the screen then finding the front windshield defroster icon and tapping it twice. All while doing 75 trying not to crash. Then as soon as it was defogged I turned it off since I don't need the "HI" setting blasting air the rest of the trip. As soon as I turned it off, the windshield fogged up again, so I repeated the process.
I don't understand why the only option to defrost the windshield is to have it on high. Every other car has the option have the windshield defrost set at numerous speeds so you can leave it on to do it's job without sounding like you're driving in a wind tunnel.