Had lots of heat, but now none

[mingus1]

I'm sure this has been covered, but I can't find an answer to me specific problem. When I bought my '82 300D a couple of months ago the heat was full blast all the time - I could not adjust the temperature. When the cabin got to warm I would turn the heat off. Just about when it got cold enough that I would not mind having maximum heat I stopped having any heat at all....well once in a while for about 3 seconds a little warmish air comes out, but otherwise it's just cold air....maybe almost lukewarm air at best.

• The fan works fine and blows plenty of cold air wherever I want it too.
• The auxiliary water pump seems to work fine.
• The pushbuttons on the ACC seem to do what they are supposed to.

From what I have researched it does not seem like it's the ACC and that the thumb wheel temp control does not usually go bad. So, where do I start?