Dear Barry S.
Before spending money on relays do the following: Depress the brake pedal with a stick and wedge it between the front seat to hold it in place, put the shift lever in reverse, put the key in position II (On) and test for power at the connection at the rear light panel. If no power check for corroded/loose ground connection at the panel, and check all appropriate fuses. If you have power check the printed circuit board. They are notorious for wearing out where the incoming power connector attaches and also where the bulb holder connectors press against the circuit. Every Volvo I have ever owned has had finiky tail lights but I have never heard of the bulb failure warning relay being the cause of lighting failure. In fact the bulb failure warning relay is usually accused of being too sensitive and warning of failure when there is none.

Good luck
Len K