The tubing behind the glove box was on my (late, great) 85 300SD. It, and the small blower it was attached to, pull cabin air across the temperature sensor in the headliner. It has less to do with actual temperature than how quickly the automatic controls react. Before I replaced this small blower I would roast until the headliner and sensor slowly warmed up from contact with the hot air blowing up the windshield. Then things would settle down.
From the browsing I did in the service CD I think the tubing/fan was a later addition; earlier W126 didn't show it and when I went to the MB dealer the parts man could not locate the small fan on the computer. He tried some other year/model, found something close, brought out the part and voila! it was the correct part.
Somewhat off topic, my Suburban isn't making enough heat. Head gasket leak is putting air bubbles in system, and air just doesn't heat as well as coolant.
It's rebuilt engine time...
I wish others had a shop forum as good as this one, and I'll try to make my Brand A and Brand G postings related.
All I want for Christmas is no more automobile challenges.