Sometimes the heater seems to be stuck on 90 degrees and you cannot adjust it. Below is how to fix the problem.

You will need to remove the climate control (dash unit) and open it up. In the picture below you will see the circuit board that is behind the buttons.

You need to clean the contacts. You will note that there is a black plastic cover over the grey rubber contact pad. The black plastic cover is heat staked in place making it hard to clean the contact.

My solution .... with a knife or small screwdriver raise the grey rubber off the circuit board...do this away from where it is heat staked. Spray contact cleaner in the gap.
Do this in several places.

I then take a piece of typing paper, copy or computer printer paper would be the same, and cut a strip about 3/8 inch wide. Slide the paper between the circuit board and the grey rubber.

The paper should get damp with the contact cleaner...if not add more cleaner.

Position the paper under a button... push on the button with your finger and pull the paper...this will wipe the contacts.

Repeat the process on each button.

When finished, remove excess contact cleaner with a paper towel. Reassemble and your problem should be solved.

Thanks to Barney Eaton for this information and the photo.