I had the same problem with my E-320 headrest. It turned out that the motor electrical contact is not good. If you take off the seat's back, wiggle your motor around until you can get use the switch to move it up and down (get someone to help with the switching). I used electrical tape to force the contact. I've been working fine for two years now.
There may also be a plastic piece that hold the headrest gear being broken, you can use a couple of tie wrap to hold it in its place.

Good luck
JeffreyP