Headrests are done.

Thanks in part to the advice I got from this site, I have just completed the replacement of the “adjusting rail” on both the headrests in my 92 300E.
The adjusting rail is the name used in the service manual for an assembly that is much like a rack and pinion. It has a toothed track or rack in which a gear unit rides up and down as you operate the headrest motor. The gear unit has a 2-piece snap fitting into which the left shaft of the headrest snaps. It is the snap fitting that broke on both my seats, thus preventing the motor from moving the headrest up and down.
Some observations:
1. As noted here, it is not too difficult to replace this part. The difficulty is to remove the panel on the seat back to get at the part, and to reinstall the panel without damage. One reason it is difficult is that it is damn near impossible to understand how it comes apart until you have it apart, unless you have seen it apart before! Even with the comments here plus the shop manual, I did not realize how the upholstery on the sides of the seat wrap around the back panel and tuck into its edges. Once you see that, it does make it easier to remove the panel. But even having done it, I find it difficult time to describe that upholstery tuck and how the rear panel fits into it.
2. As well as I can describe it, the easiest process to remove the back is as follows. Undo the two screws on the bottom, then slide the panel down about an inch, just enough for the top lip of the back panel to clear the top edge of the seat. (The back panel has a lip that slips under the lower edge of the upper edge of the seat back.) It takes a fair amount of force to move the panel, and it is difficult to find a place to grab the back panel. When I did the first seat, it took two of us to move the panel down. When I did the second one, I was able to do it alone. One way to do it, if you are careful, is to lean over the seat from the front and slide your hands into the pocket on the seat back. Press down as evenly as you can (so you do not damage the upholstery). Once there is an inch or so of clearance between the seat back and the top surface, slip one edge of the seat back out from under the tucked-in side upholstery. I have a plastic body or upholstery tool that was invaluable for this. It is essentially a tapered, thick shim (that is, not sharp enough to cut anything). Compress the seat side (a second person helps here) to reduce the tension on the upholstery. Slip the tool up under the side upholstery where it folds into the seat back’s groove, and gently but firmly lift it out. It will lift out and flip around. Work up and down from where you start. I agree with G-Benz who said that you only need to remove one edge of the panel to do the job. That makes it much easier to put the seat back together. When I did the first seat I took off the whole back, and wish I had not.
3. I wish I had taken pictures!
4. When I reinstalled the back panel, I found I could force the panel up gradually by heaving up on the bottom outer edge where the metal rails are. Use the upholstery tool to keep the side tucked in and smooth. Watch out for the side upholstery bunching up.
5. It can be a fairly demanding task on your back and neck.
6. Watch out for grease from the seat mechanism. You can get it on the upholstery if you are not careful.
7. When I opened the driver’s seat, I found evidence that the adjusting rail was replaced once before. This is discouraging, in that it indicates it breaks easily. This is one of those situations in which a complicated, expensive mechanism can be defeated by the failure of a small, inexpensive part. And the labor to replace the part is considerable, in comparison to the part’s value.
8. I suspect the trick to preventing the failure is to be very careful when removing the headrests, and never forcing them up or down manually except to remove them. Yanking up on them without releasing the latch per the owner’s manual procedure will readily damage the snap fitting at the top of the adjusting rail. When I had the mechanism apart, it was easy to see why the headrest had to be in the top position when it is removed – only there does the latch mechanism release the headrest post from the snap fitting.
Thanks again to those who make suggestions, and best wishes to others who need to do this job.

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DavidB29
1992 300E with ASR
35 years of Diesels until now!