Yesterday I looked at the mechanism that adjusts the headrest: There is a small motor with a gearbox that drives a small gear that engages a rack which moves the headrest. A potentiometer attached to the gearbox sends a signal to a computer chip. That determines when to shut the motor off when the headrest reaches the end of the allowable travel. If it would not shut off the motor, it would break the weakest link. The weakest link is the plastic rack. It broke. The potentiometer signal is also used to position the headrest to a saved position using the "save" button on the door. Because the computer chip has failed, the motor doesn't shut off. I have disconnected the motor and I'm positioning the headrest by hand. Until now I have never appreciated the simpler things: I'm so very grateful now that the zipper on my pants was not engineered by Mercedes.