listen at this experiment
Donald. Today I went to visit my father in the 300D (80 miles each way) and I disconnected the a/c supply hose to the far right passenger dash vent and rerouted it into the "return" air intake of the a/c system. I was able to get 42 degrees F out of the center vents during the whole trip. Remember - this is a 1985 model that is running 134a. The compressor did cycle a couple of times and I assume it was because of the evaporator temp sensor doing its job. The front seat passenger (oldest son) did not seem to mind not having a vent because the car was quite cool.