There isn't a lot of vacuum control in a European heater-only-system...
A lot of the W124s in Europe suffer from either a broken heater control or broken magnetic-switches. Either your control panel is broken (the part with the temp.setting wheels that is situated behind the center console panel) or the magnetic switch under the hood that opens/closes the warm water lines (left and right is controlled separate) that is situated just in front of the battery is broken.
My guess is the magnetic switch...

The center air vents blow cold air regardless the temperature settings you made.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)