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I have a 1996 Mazda B2300 and I am having problems with the heat/ac. I only get heat out of the vents. I get heat even when the selector is in the off position. I get heat when the ac or max ac is selected. I think I have narrowed the problem down to the door that controls the heat and ac. I was messing around with things and then the ac started to work, only ac and ac all the time. When I took the truck out for a test drive I could hear a door closing somewhere in the cab. Is the door in the compartment where the heater core is? If not where is it? I would appreciate any help someone could give. I am not much of a mechanic so I hope this is something I might be able to fix myself. I took it to the Ford dealership and they told me it was a valve on the heater core that was bad and they wanted $800 to fix it.  

The door that controls the heat and ac is located somewhere in the
ducting.Locate the heater box and blower motor, more than likely this
door is somwhere just beyond the blower motor it is probably operated
by a vacuum controlled actuator.Look in the area behind the glove box.
Behind the heat/ac selector conrol head {that you select by hand} is a vacuum selector switch that controls the vacuum to the actuator thus opening and closing that door. Any vacuum leak between those points
or a bad/faulty selector switch will not allow the door to open.
When you select the heat position the door should open fully letting
the blower motor push all the heat into the cab on the other hand if ac
is selected it should fully close the door so no heat is allowed in and
the blower motor can blow only cool air into the cab.
I am not familiar with your particular truck so I'm not saying that is
exactly how it works but your set up should be similar.
On some older models, such as a 1980 Buick Regal,there is a valve incorporated into the heater hose line that enters the heater box that when the ac is selected it shuts off the coolant going into the heater core making it "bypass" the heater core so there is no hot coolant in the core. So the blower motor can only push cold air out as there is no heat present in the heater box.
But this may be an older design that has been eliminated.
Regardless I would encourage you to do some searching before taking it
to a mechanic.
I also believe that Ford dealer wants to take you to the cleaners
for such a ridiculous amount of money.
Hope this helps and good luck !