The rear foglights were implemented in Europe just for what the name is....They were intended to warn approaching drivers that there is a car in front of them. They have fog overthere on a regular basis, also I was told once it had something to do with the autobahn.

What I did with the extra slot in each of the rear tailliaghts:

Find a broken rear taillight housing, disassemble the metal strip holding bulbs, remove a socket, install the socket in the missing place on the US assembly. I soldered the wire to the brake light wire so the car now has 2 brake lights on each side instead of one.

I figure that there is no sense in using the extra slot as it was intended (rear foglight), since probably 95% of US drivers have no idea that there is such a thing. They may just think you are driving around with your brake lights on!