Check fuse 6.
Do your map lights work?

 

looks like 7 & 8 feed courtesy lights to me

 

Front map lights are good.

Fuse 6 is central locking
Fuse 7 is radio/amp
Fuse 8 is light switch/ immobilizer

according to the label. All the fuses look solid.

 

Fuse 7 in the Main Fuse box ("A" - BIG fuses ) provides constant battery power to fuse 15
in the distribution fuse box ("C" - small fuses ). If both of those fuses are OK,
then the map lights will work.

If the map lights work, there should be 12 volts available at the rear seat reading lights.
The switches on those lights should select and provide a ground to the lamp from either a
controlled output from the cental locking module or a constant ground that will turn them
on any time. Check all three connections to be sure that they all do the right things.

Download the full diagrams and refer to the
interior lighting diagrams listed in section 35.
The full diagrams are available from MVS:

http://www.matthewsvolvosite.com/volvo-wiring-diagrams