I agree with Mike. You must add new solder to the joints.

Model year 86 was the first year of the North American W126 to have electronics in the pushbutton assembly. Before that it just had switches and relays.

On the 86, resolder the joints of the pushbutton switches and temperature control potentiometer. Also look for cracks on large components such as the regulator IC, driver transistors and driver IC's.
Also there are 2 capacitors that sometimes leak. They are 33uF/16V (one on each board).
Look for signs of overheating of components. A seized aux. water pump or shorted monovalve can damage it's driver transistor in the push button module.

As a side note, if the numbers on the temperature wheel are not so nice (they separate or peel off), they are available as a replacement part for a few dollars.