Mercedes-Benz 300 TD-TURBO DIESEL 1982 Owner's Manual page 36

Autom atic Climate Control
U Defrosting
H Defrosting
Independent of the position of the temperature selector
wheel and the blower speed setting, optimally heated air
will be channeled to the windshield and the movable
nozzles (2).
Important!
To ensure proper operation of the automatic climate
control system, engage buttons O or ^ at least once
a month for a short period when the outside temperature
is above + 2° C/ + 36° F.