looking at the rear of my s40 when the reverse headlights are activated, should only the left side be brighter?

If by 'reverse headlights' you mean rear fog lights, then yes. Only one side should light up, per my 2004.5 owner's manual.

And if you are talking about the bright red rear fog lamp showing upon one side of the taillight assemblies, it should on be used only in low visibility conditions such as, well..., fog.

I've seen lots of cars driven by totally clueless owners who are running a rear fog lamp in perfectly clear weather (usually because they have turned on the front fog lights in clear weather and their cars also illuminate the rears at the same time! Idiots.)