pavinjo

Actually it was easy to detach the back plate of the climatic control but i was clumsy.
First I removed the center switch (fan speed) cover by pulling it out with a small flat screwdriver into the existing recess.
Inside of the knob there are 2 small black clips holding onto the white plastic ring. In my case they were broken and since their purpose is to keep pressure of the contacts  between the slider and back plate, the contact was not fair and therefore some fan speeds were not working.
Furthermore due to poor contact there was some arcing which build up some carbon deposites on the contacts but same were cleaned with some fine sand paper. Originally the contacts were covered with some grease (should be some dielectric type) but since i had at hand only some lithium grease i used that one even that one should not be a good choice.
In the end good contact was reinstated by a M3 bolt trough the center of the fan switch.
For the time being the fan switch is working correctly.
P.S.something interesting,  while the Climatic control was not conected to the car, on making contact in the ignition with the key, main radiator fan always went on.