I had same issue. Old button was found alongside the seat. On mine the end post of the switch had been broken off. I had a spare left switch. I found upon disassembly that the headrest switch was screwed to the circuit board and could be transferred into the right side switch. Needed very small tools. Minor but annoying issue resolved.

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Hanno
'79 6.9 Sold (after 27 years)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon