I have a 1993 190e 2.3. The heat will only work when it warms up over 12 celcius(50f) outside. The blower and temp gauge works as do all of the control buttons.
I have flushed the system many times and it seemed to help a bit.
This included a steam flush through the heater core.
Sometimes I get heat out the top vents but not the floor vents,just cool air.
This is basically erratic and I never know when I will get heat.
I will say though that it works perfectly well when warm outside.
No mechanic seems to know why unless they do a expensive exploratory. Help. Freezing in Vancouver