WARNING: This page is about a different car, the 2002 Volvo C70, 2001 Volvo C70, 2000 Volvo C70, 1999 Volvo C70, and 1998 Volvo C70. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Seat Belt Buckle End Mounting
Courtesy of VOLVO CARS OF NORTH AMERICA.
Press in the catch.
Remove or install the seat compartment and secure or unsecure the SIPS airbag.