Its not moisture in the AC vent it night be on the outside. I have faced similar issue. On a rainy season drive we tend to close windows & use defogger as you use defogger, the windshield becomes cold contrary to the temperature on the outside will be slightly hot which causes fog or moist kind near to your wiper blades (I hope you are referring to this one) so simple use the solution from wiper mechanism to clear that you may have to try it many times if you are driving for long distance. Another workaround is keep ac always on (Not in defogger mode) as usual so that air circulates & maintains same temperature as outside. Hope this helps fellow TAI
members if they face such issues.