W202 rear headrest release is partly electrical and partial vacuum operated.
The switch on the dash is electrical, under the rear deck is an electric operating vacuum valve that has to click (sound is faintly hearable) after which it will allow vacuum to release the headrests.
If there is a small vacuum leak, you should be able to release the headrestst by pushing the switch a little longer...if this does not help, you need to check whether you have an electrical or vacuum problem...You can get to the control valve after removing the first aid kit with compartment box. From the valve 2 vacuum lines will go to the headrest release solenoids and 1 vacuum line to the front of the car...it is connected at the vacuum clustering connector under the black plastic cover at the right side under the hood (white vacuum line with or without a colored stripe).
with a minor 600+k