If you wanted an OEM setup for the rear fog, from what I can tell for the smart you'd have to
1. Get the european tail lights that have the combo brake/amber turn signal in the top cluster and the bottom will be rear fog on the left, backup light on the right.
2. get the european rear fog switch
3. probably modify the central electronics to allow it to work (in the same way you'd modify them to change the temperature readout to centigrade or the clock to 24hr like in europe)
Rear fog lamps are completely legal in the US and Canada, they come to the US OEM on most Audis, Volvos, Mercedes and a few old Oldsmobiles from the 90s. I've done many rear fog mods on my Volkswagens which do not come standard w/ a rear fog in North America. The newer ones require a modification to the central electronics system to allow it to operate... a huge PITA, older ones just require getting the right switch, running a wire and a bulb to the tail light cluster.
You probably won't get much help from smart NA because the european tail lights don't meet the out of date DOT requirements and they won't want to change the central electronics.
I have my smart cabriolet on reserve, when it comes this is one of the first projects that I want to tackle
So, having said that does anyone know where you can order european or aftermarket parts for smart?