Unfortunately the latest parts catalogue I have shows only the 2014 on +4 chassis harness as below. You will need to zoom in to trace wires.
It could well be that the 4/4 of that year could be the same with regard to lighting at least.
If it is there is dedicated wiring for spotlights already in place. The relay for these (A7 on above) may be missing though with an empty socket for same if spotlights were not factory spec'd. This is just speculation on my part though.
If needed a suitable diode relay of current rating to comfortably cover the spotlights current draw needs to be plugged in - typically 25 or 30 amp plug-in diode type to match existing ones. The power feed cable from relay to spotlights should already be in the loom and will be taped out of the way, probably near front edge of offside inner wing. Local chassis earth's can be added to complete spotlight circuit.
The spotlight relay is energised by a switch on the dash. Diagram below shows this (position G4) although again as +4.
If your car conforms to above diagrams I wouldn't be surprised if the redundant switch you refer to is actually for the spotlights and already wired to energise relay.
Just noticed posts above confirm my surmise.
ETA: you can buy standard 5 pin diode relay here
if needed although you may have local source. Order without bracket if fitting into vacant socket, otherwise with to allow custom mount.