If windows and mirrors are very cold, condensation can still form on these surfaces. And if your bathroom is sealed tightly, replacement air may not be entering the room fast enough to displace the moist air. Be sure to undercut your door sufficiently to draw fresh air into the room. Fan placement can also be a factor. Your fan should be located far from the replacement air source to ensure the moist air is drawn out first. Finally, too many twists and turns in the ducting will significantly reduce the ability of the blower to remove moist air. Make sure your ductwork is as short and straight as it can be, with gradual turns rather than tight corners where necessary.