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 Open Socket Housing
Courtesy of VOLVO CARS OF NORTH AMERICA.
Open the socket housing secondary locking by turning the two rings shown in the illustration. A "click" indicates that the secondary locking has disengaged.