2003 XC90 dashboard message showed brake light warning. On examination, left and right rear brake lights were not working, however the center hatch brake light did work. Replacement of the light bulbs purchased at local parts shop did not fix problem, but there were no problems with the bulbs. Resolution: Review of forums on-line suggested 2/79 brake relay on 3rd position down on right hand side of REM (rear electronic module)could be source of problem as this controls the left and right rear brake lights, but not the middle one on hatch. Went to Volvo dealer and purchased 2/79 brake relay, part #9441161 (black w 5 male spades and no diode) for ~$20.00. Access to REM on rear left just inside ofrear hatchrequires removal of rear floor panels, grocery hatch, inserts and jack plus boltsat foot of panel over REM. Strangely enough, the existing relay was part #8663490 (in blue w 4 male spades and diode). However,replacement with part #9441161 fixed the brake light problem.
Any idea why the part # would change?

 

Most likely just an updates version of the relay.

 

I have the same problem with my 2001 S80 with 270,000 miles. At first it was intermittent, and now the two lower lights stay off completely. I finally think I have figured this out. Now I just have to locate which relay is the one for the lower brakes.