If you don't find any problems with the switchover valves, then it's likely to be one of the rubber diaphragms in the actuator pods. The defrost pod is located to the right of the main plenum and can seen by pulling the glove box and looking to the left and up a little. Fortunately, on the 126, most of the pods are very accessible. You will have to order a replacement through the dealer as it is considered a "long throw" actuator and there are no replacement diaphragms that I am aware of.

(BTW, I am not aware of any memory in the pushbutton controller for this model. I've had mine apart a few times and it's relays, switches and resistors technology.)

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )