There are several posts to this problem...I have replied to almost ALL of them...
Seems like it's time for all W124 headrests assemblies to go bad right now...
Just did this repair about three weeks ago...the "lift bridge" is cheap...ordered 2 (one for later if and when the other one breaks) for $35 @ FastLane. So I guess they are $18 apiece?
You can't tell by feel or otherwise that anything is amiss until you gain access to the broken part. Normally the plastic flange that grips one of the headrest shafts has cracked. The motor inertia will still allow the headrest to be "pushed" upward, but there is nothing to grip the shaft when in the downward mode.
Anyway, here is the link to the thread I posted when I did the repair...
http://www.peachparts.com/shopforum/showthread.php?threadid=43301&highlight=headrest+repair