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 Ball Joint Nut
Courtesy of VOLVO CARS OF NORTH AMERICA.
Blow clean around the ball joint with compressed air.
Generously spray the ball joint and the screw and nut retaining the ball joint with rust penetrator.