I don't have my service manual nearby, but if I remember, you can remove the glove box to gain access to part of the right side of the heater-evaporator. There is a white plastic cam, driven by the Mode Control motor, that adjusts the various output doors. Once the motor is removed, the cam can be manually turned to direct the output air to vent/heat/defrost. Determine if there is a mechanical bind in the full travel of the cam before you look further.
Good luck.
Bill