I repaired my one at the rear yesterday.
Passenger side was working, driver side not, but a heard a short click in the motor.
Swopped headrest with motor from right to left and left to right.
Passenger side worked with the driver side headrest.
So I concluded it is not a wire, fuse or relay, but must be the motor.
Removed the motor (2 bolts), tested it under hood directly on plus pole and ground. Just a short click again.
Turned it by hand at the drive shaft, tried again with power. Started to run.
So the motor got stuck inside because in the rear it is very seldom used.
Let it run 20 times forward and backwards to run in again and to get oxidation out, works like a charm now again.