On the 1981 300SD there is an evaporator temp regulator that controls the A/C compressor on-off. Should cycle at 36-40deg evaporator temperature.
As far as the rear vent operation goes, try search as that has been "hashed-over" many times! This vent is ONLY for A/C or fresh air. UNDER the seats are heater ducts that allow heat to the rear seats.
MERCEDES Benz Master Guild Technician (6 TIMES)
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190E 2.3 16V ITS SCCA race car (sold)
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