1) If the ONLY airflow you ever get is out the defroster and the end dash vents, that's a symptom of zero vacuum present... check for vacuum at the top of the 7-port manifold (with the engine running!). If there's no vaccum, none of the flaps will work.

2) As Jerry said, you can jury-rig things to make it a little better, if you don't have time to pull the dash. I swapped the center vent with the diverter pod to get cool air out the center dash vents... drove like this for 2 years before I finally pulled the dash and replaced everything (don't just change the bad ones, change them ALL.)

3) DO NOT get the rebuild kits - pretty sure I explained why earlier in this thread. Get NEW pods.

4) To pull the dash, you need the factory service manual procedure... Group 68, job 100. Here's a link to the PDF file online (3MB download) :
http://mercedes.braingears.com/124_DISC2/Program/Chassis/68-100.pdf