I don't know why they only put one fog light back there. Jaguar, Land Rover, Saab, etc. have 2. So I decide to "activate" the other one. Easy enough to do... remove the right socket, remove the black plastic plug in the tail light assembly, run a 14 gauge wire from the lower left connector (green wire, drivers side) and using a quick disconnect plug it into the lower left (open) slot on the right connector.

Now for those who think that two lights are just too bright, you can choose from other bulbs that will fit in the socket. The tail lights are a #67 (8 watt) bulb. The fog light is a #1156 (26 watt) bulb. So if you want something about halfway, you can try one of these:
#1003 (12 watt), #93 (13 watt), #1093 (15 watt)
Or if you're the obnoxious type, try the #1295. Its 37.5 watts!!

Here's a pic of mine with both lit up using the #1156 (Yes, I know two of the tail lights were out, I didn't have any spares at the time)

 

There is only one rear fog light for a reason.This light is just as bright as the brake light and if there is two of them lit a driver sees them while catching up in heavy fog could distinguis between a car driven at regular speed and will not think the car front of him is hitting the brakes.The idea of having a rear fog light is to avert accidents and not the opposite.Thinking that the vehicle you are catching up to fast is actually braking hard will make you react a very bad way and will most certainly cause an accident.

 

They put 1 bulb for safety reasons. Volvo = Safety. The 1 bulb lets people behind you see that the car is there and the bright light is the drivers side. Two lights can be confused as brake lights on.

 

I've been driving a Land Rover for years now and it's rear fog lights are just as bright as the brake lights. I haven't had any issues running them. Same with the Jag I used to have. If there's a problem with 2 rear fog lights here in the USA, then I think DOT or NHTSA would have banned them by now. You can always use the OEM 8 watt bulb if you just want to be seen better.