If that's the only thing I have to go on, I'd start with the HVAC control head. The mode knob on your Malibu works off of vacuum. It's basically a rotary switch that routes vacuum to different hoses. The hoses are then connected to vacuum actuators consisting of the vent door actuator and defrost door actuator. The defrost door is linked to heater door so that when one opens, the other closes.
I need some help replacing the plastic levers. Did you pull the whole dash to get to them? I've pulled everything but the dash, but it appears that the lever is molded to the arm inside the vent box and that I'd need to pull it all. Any help would be appreciated. Thanks