Was driving on interstate coming home from Texas when we encountered a winter storm that was producing freezing rain. We were running the defroster on high to stop the ice buildup that was occurring on the windshield when the air suddenly stopped blowing through the defroster vents and diverted to the floor vents. Our safety and others was compromised by the ice being able to build up and significantly reducing our visibility effectively causing us to have to stop on the offramps to de-ice the windshield. Against my better judgement I did not take vehicle to the dealer because other drivers of similar vehicles had reported that they have experienced similar results in similar conditions. There were no warning lamps or prior symptoms associated with this event.