Not sure about the older models but on my W124 it's all wzywig (what you see is what you get). Simply look at how it goes up/down. Move it with your fingers. Lube all the joints with the very light Mercedes grease (which is VERY expensive) or other light grease you can get from NAPA in a small squeeze tube for $6 or so.

The problem is probably dirt that's preventing it from working right so clean the entire area with a Q-tip, then lube it.