GM-GMC: Heater controller on my 89 GMC Sierra, gmc sierra, defroster

Question
My pickup isn't blowing heat thru the heater ducts or vents. It does the instant I start my truck but within a couple of seconds it just ends up blowing through the defroster. When I turn the key to the on position without starting I hear a whirring sound from heater box and I also hear the same when I shut the truck down. It goes for a few seconds and then shuts off. Is this controller electronic or is it vacuum? My dash heater controls are the electronic ones and the truck does have a/c....please don't tell me I have to tear the whold dash out to repair this...I do don't I""""

Answer
That particular system is a tough one to work with.  From past experience, if the actuators don't receive a signal and have defaulted to the defrost position it is usually due to a control head malfunction.  It's not very technical but the first thing that I would do is swap the control head with a known good one.  (If you are trying used one's, try more than one) If the mode function still doesn't work then I would start on the diagnostic road.
Like I said, it's not a very tech savy solution but it should work.
Hope this helps