I know that there is a button behind the front seats that you have to push to get the headrests off. If you get in the rear seats and look at the rear-end of the front seats, you should see a little indent in the leather. This is the button that you have to push to get the headrests off. I have never taken the headrests off in my 1994 E420, but I know that you have to push that button sometime in the process of taking the headrests off. I hope this helps.
1994 E420 (W124)
1968 250SE (4 speed manual)