The rear heater vents are under the front seats (I knew I shoulda taken a picture while I had the seats out!!!). You might be able to see them if you raise the seat as high as it will go and put it full forward. On the 560 (or any with seat heaters) the relay for the seat heaters attaches to the bottom of the vents. The ductwork for the rear vents are those large flat plenums that run under the carper. They actually go up along the tranny hump, where they mate to the duct coming out of the heater box.
The rear center vent is attached to a "Y" plenum which runs up into the dash. The control flap is located there as well, and is operated via a two piece cable. I think this plenum attaches at the evap box, which might account for some cooling, but mostly all that comes out of the rear center is fresh air.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.