When my window got foggy I turned on the "defrost" on heat. The problem is that the vent will not get off defrost and stays on defrost. I'm trying to get the air back to blowing towards the driver and passenger but it wont do that. The knob for the vent control still turns, but the air conditioning will not blow where I need it to blow. When I try to turn the knob to get it to blow somewhere else, it make a motor sound, like gears grinding. what is it? how can I fix that?