There I was trying to figure out why I was not getting heat in my '81 300TD.
The circulator pump was running and the heat valve was opening and closing
when I had the temp to hot and was pulling its connector off and putting it
back on.  But, still no heat.

Then, I noticed a black wire in the same sleeve as the wires going to the
heat valve that went past the valve and into the depths of the engine
compartment.  I followed this wire and found an alligator clip on the end of
it that was attached to a bracket mounted to the engine (apparently for a
ground).  If I unclipped this wire from the bracket, all of a sudden I
started getting heat in the car.  If I put it back, the air started cooling
down.

So, I now have heat, but have a wire not attached to anything to make it
happen.  The temp control in the car appears to still work, as moving it
toward cold will allow the air to cool down.