W107 Heater lever adjustment

[GermanMarque]

Can someone provide the method to adjust the vertical heater/cold air controls on a 107? The passenger heater control works fine, however the driver's side now suddenly has stopped short of the bottom/off position and hence I still have heat from the vents. Don't want to go ferreting around behind that wood panel, should I disturb some other beastie that will rear it's ugly head and cause more grief. Are the heater slides vacuum or cable operated? They say if it ain't broke, don't fix it. However it seems what ever you happen to touch when fixing something else goes out in sympathy.
These are the original vertical slide operation heater/cool air controls, not the later climate control system. Thanks in advance.