Hi,

According to Volvo S90/V90 wiring diagram the tail lights have 2 fuses, one for right and one for left lights. The fusebox is located within the engine compartment near the strut tower. Only one big fusebox there you should easily identify it.

The fuses are labelled 10 and 11. Fuse 11 supplies to the brown-white wire (not brown-pink according to diagram) via a sensor box for the right tail light. According to diagram the right tail light is the one you see standing at back of car facing forward. Fuse 10 supplies to the red wire (also via sensor box) for the left tail light. BTW tail light refers to the lowermost light in the lens cluster.

Both fuses also supplies the front parking lights for respective sides. If your tail light doesn't work then your front light shouldn't work too on same side if the fault lies in the fuse or fusebox's electrical terminal. If your front parking light works then there might be issues at your sensor box located in the trunk. Just follow the tail harness and you'll encounter a box. Check its electrical terminals as the box may not be at fault too.

BTW I don't own this car. These are just from the wiring diagram. May these be helpful.

Regards,
Amarin.