Although I do not own a W211, I did own a W210, the previous model. In that car, the rear head rests did not "pop up", they only go down. In other words, you had to manually lift the head rests to the raised position. The button for the rear head rests, when pushed, would only lower them. Try raising them by lifting them up by hand. Then, with the ignition on, push the button and see if they drop back down. I'm afraid I don't have any ideas about the front ones.