No socket on the back of a class lights.
You will need a multimeter to work out which is the fog light wire and spur off that.
Take the lamps out of the original unit once you have fitted your other fogs , as running two sets of fogs melts the contacts inside the multifunction light switch and a new one is around £100.
I did ours when I fitted the brabus kit , fiddley and time consuming , but worth it