It cost me about $50 to enable the rear fog lights. European headlight switch control #61319134726 ($40 from UK with shipping Ebay). Two micro terminal sockets #61131393704 ($1.19 a piece) and a length of 05mm wire #61126908726 ($4.49).

Remove original headlight switch with the trim piece
Disconnect wire harness clip from the back of the switch.
Remove switch from the trim piece
Remove plastic board with all the micro socket wire connections from the harness clip (small tab on side of clip)
Find position #9 (will be empty) this is where the first micro socket is inserted
Remove driver side kick panel in the footwell
Locate Light Control Module right above footrest on the left side of the firewall (module is encased in a black protective box, it will slide out)
Disconnect the wire harness from the module, there are 2 plastic socket boards where the wires terminate (brown and blue)
The blue one is what you need, find position #33 it will also be empty.
Take the new wire and snake it from the headlight switch behind the dashboard to the module wire loom harness
Terminate each end of the wire with a micro socket. Strip about 1/4 in of the insulation, twist copper wire and insert into the micro socket with enough insulation still left so that using a small needle nose plier, you can crimp the neck of the socket around the insulated part and then crimp the exposed wire also inside the socket.
Push each socket into the designated empty positions until it clicks in place. The small raised 'v' on the end of the socket should be visible in the little window on the side of the plastic board at each end.
Reinstall all the wire harness boards back in the plastic clips.
You can zip tie or tape the new wire along the harnesses.
Place module back in its box and slide in place
Install footwell.
The plastic trim for the headlight switch will not accommodate the new switch, since there is an extra button now. You have two choices, you can buy the trim piece for the new switch or what I did, for now, was to carefully and gently follow the contour of the new switch position which is already outlined in the back of the old trim piece. Score along the plastic raised contour until you can neatly remove the excess bit.
Install new switch in the back of the trim piece you just modified.
Connect wire harness clip to switch.

Turn on the car and check out the fog light bulbs that have been in the taillight since day one, but were never turned on until now.

I recommend testing the switch prior to putting all the trim pieces back in!

 

What is the model and year of your car with the hidden rear fog lights?

 

2010 e61 N54

 

Not sure if I have what you have, I'll have to look a little more closely at mine. It's an 08 535 e60

 

You can use this vid for reference (not mine)