The Prius assumes you want to go to Auto mode with A/C when you kick off the defroster. I just keep it on auto but turn off the A/C, and hardly ever use the defroster. But you can only do this through the MFD (Multi Function Display). I've used the defroster exactly twice since November as I like a cold windshield as snow doesn't stick, but that's just me. As long as the air continues to move around I don't have any problems with inside condensation oddly enough. As big as the amount of glass I have, the auto function (without A/C) never really got steamed up even on short trips. With A/C on during Auto it used to frost on the inside on really cold days. I found out that the A/C sometimes has condensation on it even during cold days and allows the car to steam up inside and then frost up when it gets really cold out. I found out the car is smarter than I am and just allow the car turn the blowers on and off. I just use the temp buttons to turn the temps up and down. Hope this explanation helps, even if it a little long.
