I am making a leap of faith that the S80 model year 2005 wiring is similar to 2001, as well as 2004 V70 and XC70 (I have factory wiring diagrams for all of these). I am also assuming you do not have high intensity (Xenon) lamps.

There is a separate relay for high and low (dipped in Volvo lingo) beams, but not separate relays for the right and left side. There are, however, separate fuses for each high beam and low beam filament (4 in total).

The headlight relays are mounted on the Central Electronics Module (CEM), a large panel that is mounted behind the instrument panel just to the left of the steering column.

My experience with both a S70 and S80 is that lamp sockets are of poor quality and deteriorate quickly from heat. I had to replace 3 (in 32,000 miles) on the S70 and 2 (in 44,000 miles) on the S80. Some models have a headlamp harness made up of resistance wire to reduce lamp voltage and improve lamp life. These harnesses run warm and some other owners have complained of early failure. You might try flexing the harness to see if the affected lamp goes on and off.