I live in NE GA. In winter time, the humidity is not that bad. Having said that, I have noticed condensation only after it has rained heavily. We have not had rain for the last few days, ergo no condensation seen. Maybe that is the missing link!
Albeit, when I did see condensation, the formation was heavier closer to the bottom of the windshield (i.e. where the dash meets the windshield). I am suspecting that the dash stays warmer longer after the sun sets and the air around the dash is warm too. Add humidity to this equation and this moist warm air around the dash condenses immediately on the rapidly cooling windshield.
Or these is a leak somewhere from the engine and as the engine cools, warm air leaks into the car and condenses on the windshield.
Next time after it has stopped raining, I am going to leave the window slightly open to see if this warm air inside the car dissipates into the environment. Looks like the air inside the car is more humid than the air outside anyway!
Oh well, if this doesn't make sense, I might have to hit those physics text books this evening!