1995 Chevy Beretta defroster not blowing air on windsheild

There is a vacuum or elecric motor that moves what they call a blend door and position door. It may be stuck or it may not work. You can stick your hand up there and feel around and try to move it to see which door you have a hold of. The frost may be a leaky heater core or just cold air coming in creating the frost. The cold air would be coming in due to the blend door stuck/not working.