As stated previously, 85 300SD 95000 miles with click on relay under seat. The middle two wire connector under the carpet is the power from the door mounted headrest switch. My multimeter showed voltage both directions when the headrest switch is activated. That led me to take the back off the seat - two screws at the bottom. Couldn't see anything wrong with the wiring on the left side mounted motor but scared to remove it. Still thinking it was bound/limited, while I was sitting in the backseat, I pulled slightly towards me on the vertical mounted black bars that connect the chromed bars from the headrest to the plastic gear mechanism - while triggering the switch. Headrest moved immediately back up - WORKING! Now realizing this is stuck - not burned out - I liberally used my White lightening (bicycle gear wax lubricant) and dribbled it down the openings at the upper side of the holes in the two black vertical supports that hold the bars from the headrest. The lubricant ran out the bottom so all is lubed. Now headrest moves way faster than the drivers side leading me to believe it is dry too.
Lack of lubricant is likely why these headrests stop moving! If your mechanism is clicking and not moving, try this trick before taking off the carpet springs etc. Its less than a 5 minute job.
Harold in Sequim