If you have the upgraded indicator stalk, with the front and rear fog positions, then all that is needed is to fit it, ensure the relay is in the passenger side footwell, and the fuse is on the drivers side footwell.
That will provide 12v to the rear bumper connector when sidelights/dipped/main beams are on, and the rear fog has been toggled on.
If you are trying to fit an aftermarket switch, you will need to tap into the relay directly, as the foglight on signal is sent via the ECU (this is because it's a logic state that resets when ignition is turned off. Rather than a physical switch holding the voltage high)