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 Large And Small Nut
Courtesy of VOLVO CARS OF NORTH AMERICA.
Tighten the large nut. Tighten to 40 Nm
Tighten the small nut. Tighten to 30 Nm
Check that cable is routed from the level sensor as illustrated.