The rear light circuit board inside the lamp units with the fixings for the individual light bulbs are identical on both sides, as are the two looms that run down either side of the car to the plugs that then connect into the circuit boards on the back of the light unit. If you look for a guide on here for changing the PH1 lights for a PH2 lights then you will see a guide for the individual coloured wires in the looms. Follow the fog-lamp wire, lets pretend for the time being its blue, in the driver's side loom to the connector block in the driver's side foot-well. You would then have to run a wire from the 'blue' wire to the 'blue' wire in the passenger side foot-well connector. I'd just temporarily run the wire across the centre of the car for the time being and just loosely wrap the bear ends of the wire onto the 'blue' wires in the looms. Once you're sure you've connected the correct wires and its all working fine then you can run the wire behind the dash and under the carpet and properly attach it to the looms.
I thought about doing it myself as I've gutted my 172 as I'm turning it into a track-car and re-routed all the looms inside the chassis, the extra fog-lamp wasn't worth it to me, but it’s a fun mod to make I suppose.