| The "mode doors" control where the air is directed. There are 4 vacuum motors on the heater box. The motor that controls heat versus defrost has a red and blue vacuum line going to it. Use a hand vacuum pump to operate the motor to see if it's working properly. Apply vacuum to each side, one at a time with the blower running to see if it switches modes. Also, check to see if the motor holds the vacuum.
If the motor doesn't change the mode or the motor doesn't hold vacuum, replace the motor. If the motor checks out ok, then you have a vacuum leak in one of the lines or the switch at the dash. | 
Linear Mode
