If its a 94-97 you are likely out of luck. Sometimes the cable clip will break at the HVAC control on the instrument panel.
Look on the bottom of the blower box on the passenger side near the transmission hump, move to the right, you should see the round metal stub sticking below the plastic.
Take pliers and try to gently rotate it, if the cable has become detached, you can leave it on max heat or cold (likely you are wanting a/c).
I have pretty skinny hands and arms, and have been unable to reattach one with the described condition. The dash would have to be partially rolled back, but at least the blower box remains attached and freon, coolant etc isn't lost.
If its a later truck (98-02), they have an electronic. "solenoid" control that must be properly "calibrated" or you'll have the malady which you describe. Several members have reported more r&r to solve this one too.
Good luck
Andy