I have a newly acquired 73 220D that the heater blower works at all three speeds but no warm air comes out of the vents even after driving for a long period of time. One thing that might be related is I have noticed my normal operating temp never reaches past 175F according to the gauge even if I let that car idle for some time and when driving down the highway it likes to stay in the 140F area.

I remember one of my old E30's had the wrong thermostat installed and the car constantly ran cool and the heater blew cool air until I changed it to the correct thermostat so I'm thinking this might be a similar situation?