Several months ago, my lower brake lights failed.
I've replaced, bulbs, both fuses, that shunt and.....the blue relay.
I brought it to the glitzy Volvo dealer parts department, and they told me that the current replacement was a different part #, and sold me the current replacement.
Still no brake lights (third light has always worked)
Recently, I found some older posts stating that you need to replace TWO relays ( brake light and foglight, Part nos. V08651661 and V30765030) to fix this known issue. Is this correct? Why would you need to replace the fog light relay?
At this point, I'd gladly spring for another relay if that solves this maddening issue with this vehicle

 

Fuse 3 in the D fusebox is for the outside brake lights. Note the 3rd brake light (10/19) appears to not be powered through the relay 2/79. 20/31 is the shunt and a yellow wire goes to left and right brake lights. Have you found power with your $5 test light at the yellow wire on the assemblies? Have you scanned the entire car (all control units, including the CEM and REM) for any self diagnostic codes?