The switch that controls exhaust and fresh air is broken on my quietmaster air conditioner. It looks to me like the vent is stuck open, which of course affects the unit's ability to cool. I already tried to unscrew and remove the top unit that has the louvers so I could get inside, to no avail. (In fact I think I broke a piece trying so hard to get if off!) How can I fix this, please? It seems to be a pretty vital element for the AC to work properly... Thanks.