'91 Sierra. No heat, Just cold air

I have a 1991 Sierra that I can not get heated air out of any of the vents. It has the electronic controls, that seem to be working fine. Also the heater core hoses are hot to the touch, so I know the core is getting hot water. However, when I try to get heated air for the defroster, or anything else, it's nothing but cold air. I need help understanding what to look for to solve this problem.

I was talking to the dealer today about the actuator that opens and closes the flap to change the air flow from hot to cold. Can someone tell me where this one is located and the best way to get to it, So I don't have to tear the truck apart in search of it.

I have a 90, the actuator is behind the middle of the dash. On mine, this is now a CD player but is was a tape deck from the factory. Take the glove box out and the tape deck and then you can get to the actuator.

//2000CAYukon

Is the actuator located there in most sierra's? I have a 1998 and the door is sticking.