I know this is an old thread but I'm currently up to my neck in this project and this thread has been my Bible. Anyway, it seems the center vent pod, 124-800-02-75 is NLA from Mercedes. I found one supplier online who claimed to have it, and at a really cheap price. I thought it seemed to good to be true and it probably was because they took my money a month ago but haven't sent the part.

Anyway, the advice I got from someone is that this part is the same pod as 124-800-03-75 and you just need to switch the arm over from the original part. 124-800-03-75 seems to be readily available at a number of places so maybe this will help someone.