There are tubes connected to the back of the control unit that go to door actuators. Get a cheep had vacuum pump and hook it to each actuator and see if each door opens and the diaphragms inside are not leaking if they all work replace the control unit. If the doors have broken off and are laying in the bottom of the heating unit you need to replace the whole unit under dash. On extreme 4x4 or the show trucks they had a repair kit for dodge and chevy. The doors open and close just like a cheep plastic tool box with no hinges just a piece of thin plastic that bends back and forth till it breaks. The kit comes with metal hinges and new doors
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