Sorry for bringing up heating issues. This may be my last time trying to justify keeping my 1981 300D NA.
The symptoms as I've observed them.
The heater works just fine when warming up the vehicle for the first time of the day, even at highway speeds. When it gets warm inside the problems start. At a constant highway speed/ 65ish, the temp out of the left 2 dash vents is slightly warm/ always a little above outside temp. Right 2 dash vents are cooler, close to outside temperature. When I slow down to go through a small town, the temperature from the left vents cools down to match the right vents. I can select defrost at this time and get good hot flow as expected. Returning to highway speed, the left vents return to slightly warmer condition. When in this state temp control has no effect from MIN/MAX and all points between.
Sorry to bore.
Thanks for ideas.

PR