Heater Blend Door

Have you checked to see if your heater core valves are turned on under the hood?
If they are open your coolant thermostat might be stuck open.
There is an outside air vent actuator near the right floor board behind a panel from floor to glove box.
If that door is stuck open cold outside air rushes strait to the fan and out thru the vents.

 

The Pete I worked on has three actuator behind the panel like lowboy said. one on the r/s is for fresh air and the 2 that you can see once you remove the panel work the heater bend doors, you can turn the knob and see if the motor is turning, and you also have to remove the motor to make sure that the part that turn the blend door shaft is not stripped, and double check if the door is moving freely.
One some Pete you will find a heater valve actuator which is under the hood which open and close the valve.
If you have to replace one of the actuator, make sure they sell you the correct one,otherwise it will not work, if it still has a p/n that is very help full.

 

Also the actuator that sits right behind the center dash panel on top of the air duct is for vent, floor and defrost. I've not actually figured out where the heater blend door is but ours has not worked in years as we have to turn the valves off under the hood every summer to have AC worth a toot. It must have quit with the heat on and froze in time waiting on someone to find it and fix it.

The 379 Pete in my avatar, its defrost control motor went out so every winter I manually moved it to defrost and that's where it stayed all winter. It was in the same exact location as the 357 Pete.