To get the vent working you will have to pull the entire center console. The slider on the vent is connected to a lever at the heater box in the center of the dashboard, via a two piece cable. Either the lever is stuck or the cable is kinked and that is causing the slider to stick at the midpoint.
Yes, as 420 stated, the air isn't heated as it is ouitside air (that might receive some cooling benefit from the A/C). The heater vents for the rear seat are under the front seats.
Try doing a search on "center console removal", there is a question by Mattman about the same issue and I have some pictures and tips there.
'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.