Mike, there's a release for them under the covering of the back of the seat, below the headrest. You have to feel for it under the leather. Can't remember which way it goes, though...

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Bill Streep
San Antonio
'57 190SL (toy)
'08 S5500 (mine)
'09 CLK550 (wife's)
'06 SLK350 (daughter's)
'11 GLK350 (daughter's)
'03 CLK310 (spare)