Where is the blend door switch

it's not a valve it's a blend door or blend door actuator under the dash that switches the way the air is handled in your vehicle. I would start by removing the fan motor in the passenger side foot well and make sure the fan is clear of all debris. then you should be able to switch cold to hot on the dash and hear a door change the direction of the air inside the box in your dash. if that blend door is not working properly then you will need to see if there is a blend door actuator http://www.youtube.com/watch?v=AKA3LLFreTw

Check blend air door to ensure it is moving, Did you put the motor coupling is same position if you moved blend air door to mate to motor you will need to close blend air door remove motor hook up power and using temp control set mote in the proper position

could be blend door or possible heater core is half clogged check your blend doors and also check your switches for heater control start with checking switch

Possible bad blend door actuator .some gm cars can be relearned to the new climate control head

http://www.f150online.com/forums/hvac/216009-blend-door-replacement-pictures.html

Go to this link. You may change your mind on doing this project.

Common question on this site. On late model cars, the vents are controlled by blend door motors. Small electric motors open and close doors to direct air flow.
Chances are you have a blend door that is stuck or a failed motor.
The default position is the defrost to make sure the windshield is always clear.
Someone will need to look behind the dash to see if all of the doors will move on demand.

You have a blend door issue under dash. Also possible blend door actuator motor may have failed. Try moving the mode select dial on control panel back and forth and listen for air sound change under dash at both sets of vents. If do not hear air flow change door is not moving.

Blowing hot air when set to A/C is almost always a stuck blend door, dead blend door actuator, or a cracked or disconnected blend door actuator vacuum line. The blend door is inside the a/c ducting under the dash, but the actuator will be accessible outside the ducting.

I would 99% doubt its a clogged core. Have the blend door checked (its a door that cycles between hot and cold)

check or loss of vacuum to mode control switch in dash ac control. these are not blend doors, they are air flow control, a blend door controls the air temperature not where it comes out.