Not sure about the flap for the blend feature, but thats on the list to check along with the temp sensor. I got a nice look at the underworkings of the dash assembly from a W126 I just recently removed the dash from. Got a good look at what valves, flaps and dampers look like...not sure of how they all work in conjuction with each other, but I have a general ideal of the placement of the vacuum valves.

Nothing beats hands on looky, touchy, feely! I learned something from that salvage yard expedition.