It won't be obvious until you get one off for the first time.

The seatback edges fit into a groove along the seat. The edges are held in place because of the tension on the lumbar support.

Assuming you are doing this from the back of the seat, to relieve tension, pull the lumbar towards you on either side (depending on which side you are trying to remove first) and you can coax the seatback edge off by grabbing the bottom of the seatback and pulling gently. As the bottom edge is separated, you can grab along the edge and pull the rest of the way.

Someone here posted pics on this on another thread, and there are several threads on this topic.

The prevalent cause of the headrest failure is a broken slider. When it fails, it no longer attaches to the headrest properly, so you can lower the headrest, but not raise it.

The part can be obtained thru FastLane, and goes for about $15.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car