It sounds like the mode door actuator isn't working. That is what changes air flow direction. First, take a look through this link:
Here are the directions specific to your vehicle for replacing the mode door actuator. The attached pictures correlate with the directions.
1999 Chevy Truck C 1500 Truck 2WD V6-4.3L VIN W
Mode Actuator Replacement
Vehicle Heating and Air Conditioning Air Door Actuator / Motor Service and Repair Procedures Heating and Ventilation Mode Actuator Replacement
MODE ACTUATOR REPLACEMENT
1. Remove the floor air outlet duct extension (1) from the floor duct (2).
2. Disconnect the electrical connection at the mode actuator (10).
3. Remove the mode actuator mounting screws from the HVAC module.
4. Remove the mode actuator as an assembly with the actuator cam.
1. Install the mode actuator as an assembly with the actuator cam.
- Line up the heater/defroster valve lever with the mode actuator cam slot.
- Line up the A/C valve lever with the mode actuator cam.
NOTE: Refer to Fastener Notice in Cautions and Notices.
2. Install the mode actuator mounting screw to the HVAC module.
Tighten the mode actuator mounting screw to 1.6 N.m (14 lb in).
3. Connect the electrical connection at the mode actuator (10).
4. Install the floor air outlet duct extension (1) to the floor air duct (2).
Let me know if this helps or if you have other questions.
Images (Click to enlarge)
Tuesday, November 5th, 2019 AT 7:33 PM