I don't have a W126, but I've seen this posted before.
I believe the seatback has to come off, then you'll see the screws that need to be removed to detach the arm rest.
If you're good with using the search function, you'll be able to find it.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".