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)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator