2001 S60. The right rear light bulb bracket has positions for 4 bulbs but one of the holes the bracket mates up to is blanked off. Not so on the rear left, all 4 bulbs are there. Is this is mistake in construction of the car? I can see where to punch out the plastic in the light housing, but not sure this should be done.

 

I believe it's the rear fog light. It's only on one side of the car (I assume to not confuse drivers behind you into thinking you're braking)

 

Correct. If you have them both lit up, it looks like you're riding your brakes and it's much harder for your brake lights to get attention when you're slowing down.