While at it. That reminded me of the anti-fog spray for windshield.
And there is also the waxy and oily substance that comes in small container. You dab it on cloth and spread on the windshield.
The catch is, you only remember it when it´s already raining and cold! The device you mentioned (the defogger/demister), on the other hand, is always there.
If it is malfunctioning, though, you can use the anti-fog wipes I have just written about. Better safe than sorry!