As far as I know the european W124, the rear headrests do only have a remote pull down function. They have to be pulled up by hand.
Folding down the headrests is partial electrical and partial vacuum controlled. The vacuum comes from the engine and therefore the engine has to provide sufficient vacuum to fold the rear headrests. If you have to push the button for a longer while to get the headrests to fold down, you most probably have a small vacuum leak somewhere in the system (at least, that was what I had with my C class).
I never knew that the (W140) S class rear headrests could be put up again by button...is that partial vacuum controlled as well, or is that completely electrical?
with a minor 600+k