seat is easy. there is a switch on the bottom custion . you push it and pift out the cusion.
Then there are a couple of bolts I believe holding the backrest on. you undo those. Pull out the headrests. they should just come out I believe.
Left the rear panel to gett he hooks at the top out and it should come out.
Then you have access to the headrest locks.
Perhaps the one is sticking or something. Otherwise a vacuum leak could have stopped it from closing the lock somehow. who knows. Best to gte in there and check it anyway, maybe lubricate the mechanism.
I'm thinking vacuum or something is disconnected or frozen
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.