I haven't seen any specific threads for this yet so I thought I would make one.
The BRZ/FR-S will be equipped with a rear fog light in countries where cars are required to have one (like those in Europe) and the US versions will get a non-functioning reflector in its place. The light is located in the red upside-down triangle on the rear bumper between the reverse lights:
Since US regulations don't require rear fog lights on our vehicles here, not many people know what this light is actually for. The entire purpose of a rear fog light is to make it easier for drivers behind you to judge where your car is in bad weather. It's as simple as that. It's also not a good idea to drive with this light on in clear weather because it actually becomes very distracting for other drivers on the road because it runs at the same brightness intensity as your brake lights.
Some people will try to incorrectly wire the rear fog light as an additional brake light. While this doesn't hurt anything, that is not what the light is intended for and your car is already designed from the factory with more than enough brake lights.
Hopefully this thread will help provide some info for those looking to make their rear fog light functional. Some members have verified that there is an empty spot for a bulb in the rear fog so it should be fairly easy to wire the light up.