I think your owners book may indicate Cool air only for the rear cabin. Check it out. Turn it off this time of year.
As I recall rear cabin heat is ducted under the front passenger seat only. Who knows why?

Good luck. I liked the w-126 so well I bought another.

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Buzz
1977 300D 151K (TMU) DB922 Pastel Blue w/Blue
1981 300SD 450K DB753 Astral Silver w/Black
1987 190D Turbo 340K DB199 Black Pearl w/Black
1990 350SDL 190K DB199 Black Pearl w/Black
1994 E320 Convertible 65K DB(9)744 Brilliant Silver w/Grey
1995 E300 Diesel 150K DB (5)904 Midnight Blue w/Saddle