Thank you for the reply, sounds like the switch is bad, I am sending you instructions in pdf format on how to replace the multifunction switch, hope this helps you out.
CLICK HERE FOR INSTRUCTIONS
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Thank you for the accept, based on your description it sounds like you may have a bad fan clutch, and is not dissipating the heat away from the condenser, also sounds like the R-134a may be a little low also. If the problem was vacuum related then you would have airflow from either the floor or defrost only.
Those trucks did have problems with the blend door actuators, I think you are right.
Thank you for asking your question on JustAnswer. The most likely cause of the problem is the BOO (Brake On/Off) switch located on the brake pedal lever, below are the test procedures, to determine the exact cause, as well as replacement instructions and wiring diagram.