Quoted By:
Volvos use a plasticy paper type stuff to carry all the circuits across the taillight assembly. You may need a replacement, to check:
Put the car in reverse, have someone hold the brake (or you could jam a piece of wood between the brake/clutch pedal and seat), and turn the light by the round plastic holder in it's slot, it might not get the best connection where it normally sits. Try swapping the round plastic holder that hooks onto the assembly, or even the way the lightbulb sits in it's holder.
Had the same problem with a brake light on my 760 Tarbo, always acting up.