When the heater does blow hot air (towards the windshield) but not through any of the other ducts, I think one of your valves inside the heater housing is not controlled any more (due to a vacuum leak or broken vacuum pod).
A valve has to be opened inside the heater housing to let the incoming air flow through the heat exchanger so it gets warmed up.
My guess is that this valve stays closed and the incoming air is being led straight into the interior...

greetingz,

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