WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Rear Head Restraining Locking Clips
Courtesy of VOLVO CARS OF NORTH AMERICA.
Push in the locking clip (on left side of head restraint).
Pull the head restraint out of the rear seat.
NOTE:
For S40
, move seat backrest forward a bit (L and R).