The sunshade must be completely retracted for the head rests to drop. Put the sunshade down then push it into the slot manually - mine goes about a cm further in. Then start the car and hit the button - the rests should drop like stones with a thud! They must be pulled back up manually. I've heard some people sya their's only drops when the car is in reverse - this seems like a possible "one of" quirk to me.

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1992 Black 500e
1991 Black 300te 4m
1992 Black Renntech 620sl
1979 Porsche 911sc targa