I have a similar car 85 W126 300SD with 95000 miles and my passenger side power headrest does the same click as yours. This car was my Dad's and I'm trying to keep everything working. So far, I've been under the seat and realize the clicking is a relay - black in my case - 6 posts - vertically mounted just behind the three removable wiring connectors (under carpet). I've removed all and cleaned with contact cleaner. Still clicks. My headrest is all the way down - hard down. I think its jumped the limit switch somehow and am trying to see if I can get it to move up. Does anyone know how to jump the switch and which connection would do this? A few months ago when trying to do this, I was running the entire seat back and forth and suddenly the headrest did move up and then worked fine in the middle type positions until ran down hard again by an energetic 12 yr old. Now clicking again. This seems to be a common problem. Anyone else solve this?