Yes, the rear wagon headrests are locked in the upright position when the car is fitted with the third rear-facing seat. The headrests serve as head restraints for both, middle seat and rear-facing seat passengers.
The front and rear head rests are NOT interchangeable. They use different locking mechanisms and.
I guess the only way you will find a rear headrest that moves in an angle will be fitting the model from wagon that does not have the third seat.
CL 500, 1998
S 500 L, 1998
E 320 T, 1995