I have an '85 300D - W123. I only get air out of the dash vents and defroster, like the original poster. Also, the air is only barely warm most of the time, even if the dial is set to "MAX."

I can shut off the engine, let it sit for 5 seconds or so, and restart it, and I will get hot air for 15-30 seconds or so. Then, as I drive, it will gradually cool off until it is barely warm.

Is the W123 setup similar? Is this a failing aux. water pump?