When your dashboard heater control is hard to move (or doesn’t move at all) this could be your issue. Over the years the valve starts fails due to rust and corrosion. When this valve starts to stick, it puts excess strain on the PLASTIC heater controls in your dash. You don’t want to replace those. It’s ugly, trust me, it’s no fun. OE part.
sorry, not available for pre-1984 years or those with a rear heater