I am not familiar with your model, but on a W126 you need to remove the knobs, pry off a trim behind the door opening lever, remove the one screw securing the trim, then remove the other securing the door handle, remove two other screws holding the door handle (one halfway, one near the other end), door handle will come off. Remove door lock plunger by unscrewing, remove the stainless piece near door striker, then carefully remove the door panel. I am not sure if it's press fit or otherwise, some pull straight out, some up-and-out.
Once everything is off, partially peel off the plactic sheet to expose the parts you want, the seat switch can then be taken off (two screws and wiring harness). You can shake it and if something's loose, there may be loose parts.
Mine did so I opened the switch itself for cleaning/rebuild, and fixed the headrest, but don't do it unless necessary, lots of balls and springs, so be careful!
My backrest wasn't working also and I thought the motor was faulty, and gone all the way to remove it but found it fine. Turned out that the one terminal *within* the plug that goes into the switch were not properly placed! And a simple re-do fixed it.
BTW, the wiring harness goes under the seat, check for proper contact. You can remove seat back cover (two screws at the bottom) to examine the headrest mechanism.