Hi Maui,
can you hear the motor running when you operate the switch? If so it is probably a plastic part inside the seat. I had the same problem with my 94 E320, it turns out a plastic part that goes around one of the metal arms that drives the headrest up and down that had broken. You have to take the back of the seat off by removing 2 screws on the bottom and then push down on the seatback (I did this by pushing down with my hands in the storage pocket while leaning from the front of the seat). I remember I had to push really hard and was afraid of breaking someing the force required was so great. Once I had the seatback off it was easy to located the broken plastic part. I would suggest researching this forum, as this is where I found the instructions to remove the seatback. Also, be very careful when operating the headrest, I broke mine when I held the switch in the down position too long, not realizing it was fully down already, that is when mine broke.

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1999 SL500 87K
1994 E320 112K
1998 Ford Ranger - the daily driver 274K and no major repairs (an indestructible truck)
1978 Pontiac Trans Am 110K
1977 Pontiac Trans Am 130K
1999 SL500 87K (the garage queen, only out in the summer)