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.