That's just the way it works, if you don't have the rear climate control option. You can only close off the vent at the vent itself. If the wheel that is supposed to close the vent isn't doing its job, it is possible that the cable operating the flap is out of its seat and needs to be reseated. Heat is delivered only from under the front seats.

The W140's with rear climate control are similar in that only air conditioned air is delivered to the rear system, but to get heat, the rear climate control then routes the cooled air through a set of heater cores dedicated to the rear system to re-heat the cold air. Funny, eh, but I guess that was the easiest way to design it instead of building in a whole second air intake for the rear heater.

Brett

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1997 S600 sedan