I just helped a buddy change out his bad heater core and in the process of putting everything back together must have knocked something loose. When we finished the heater core replacement, before putting the dash back together and the computer back up into the dash we tested and the vent control was working properly. After putting everything back in it now blows through all vents no matter what the control is set on. First question, is this electrically controlled or vaccum operated? Anyone have any idea what, in that area of the dash i should look for?

 

The system of doors that controls where the air blows out of the dash is controlled by vacuum. The hot/cold control is controlled by a stiff wire from the HVAC controls to the heater box.

Look for loose or broken vacuum lines. Also you might want to check the control valves on the HVAC control unit. There's two of them (3 & 12 in the diagram), and they like to break and leak. Usually when that happens (or the vacuum line is disconnected under the hood) the vents default to defrost.