'88 Sierra climate control probs...

My climate control unit does not operate properly, so I've had to resort to pulling out the fuse. The display goes erratic, the vents go to defrost position, and it will only blow cold air. None of the controls work, such as to shut it off or change from heat to air conditioning,etc. I've swapped-out the unit itself, and even swapped-out the relays behind the glove box. Answers anyone? It's friggin' winter-time and COLD !!!!!!! Thanks in advance...