I'm not familiar with C280. But on my 91 300D the electrical switch is loacted under the rear seat of driver side next to the window control unit. you can disconnect those blue/white vacuum hoses and suck on them/use a vacuum pump to release the headrests. The other way around is to realease them from the trunk area. take the panel behind the seat off, you'll see where the vacuum lines connect to, disconnect them and use a spare vacuum tube to release them. you might be able to push the locking rod back from there...not sure!
Vacuum leak is not visible to human eyes! You can spray something on it and watch it disappears...not a good way. you'll need a vacuum tester and a vac pump as well.
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