1990 1/2 ton heater controlls not switching

my 1990 1/2 ton has a heater that was working fine until it got cold.the previous owner also had trouble from time to time.when i switch the heat (defog/floor)controll,it will not go back to defog setting.the vent switch which is next to the heat switch seems to work fine.also the a/c no longer comes on with the a/c button and the off switch also no longer funtions.this all seemed to quite at the same time.it seemed to happen since the weather got cold?any ideas would be greatly appreciated.thanks

3years ago I had to switch out my temperature control module. That is the box with all the controls on it. It fixed my issues then. Now I dont have the first 3 speeds of my fan. I haven't had a chance to troubleshoot it but Im suspecting it is some cold solder joints in the same control box. GM seems to have lots of problems with that in other parts of the electronics

3years ago I had to switch out my temperature control module. That is the box with all the controls on it. It fixed my issues then. Now I dont have the first 3 speeds of my fan. I haven't had a chance to troubleshoot it but Im suspecting it is some cold solder joints in the same control box. GM seems to have lots of problems with that in other parts of the electronics
where might this temp control module be located.what year is your truck?is it right behind the dash buttons themselves?is it a big job to change?what was the cost of the part?

its the box where all the buttons and controls are mounted on. Have you ever removed or replaced your radio? Its basically the same procedure. You have to remove the plastic dash part that goes around your radio and the instrument panel. All you need to do is pull on it and it will separate. If your truck is an auto, it'll be easier to pull it off by setting the gear shifter all the way down. Once that is out of the way, you will have access to the control module. I don't remember the rest of the steps but look around and it should be obvious or get a service manual to help you out.

by the way my truck is a 96 GMC Sierra.

We are having the same problem with our 1990 Sierra 1/2 ton, when i switch the heat (defog/floor)controll,it will not go back to defog setting. the vent switch which is next to the heat switch seems to work fine and does the a/c but the buttons for the volume on the radio no longer works now.
What is the first step?