I believe that you are correct that the glovebox side pod is the defrost vent:
http://www.w124performance.com/images/W124_dash/manifold_defrost.jpg
Do you get air through the center vent when the system is cooling? If not, then I believe that the diverter vent is your issue.
Here are some more photos thanks to GXSR:
http://www.w124performance.com/images/W124_dash/
and a thread on pod replacement in the 124:
http://www.peachparts.com/shopforum/showthread.php?p=197605