Maybe your offside reversing light has been converted to rear fog, quite a common mod. Mine's been done like this.
Also I would suggest that the switch you have found is for this. Have a look at the offside reversing light, it will probably have a red film on it. If it doesn't it could still have been converted by using a red bulb.
Try switching the switch under the dashboard, with the lights on (should only need to be on side lights). Does the switch illuminate? Chech the reversing light, is it lit?