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: Identifying Seat Belt (Inertia) Reel
Courtesy of VOLVO CARS OF NORTH AMERICA.
Check that the seat belt (inertia) reel locks as it should. Test the function by pulling it out and jerking it repeatedly.