well its your choice..
All you need to do is get a relay switch, wire up 12 volt supply from battery to it, loop it out the the fogs, and put a line from the relay to your one of your side light wires.
When your side lights go on it sends 12v to the relay, which then switches the main 12v supply to the fogs, make sure you put an inline fuse at the correct rating depending on what power your bulbs are.
Pretty sure its still illegal, but its your choice.
oh, its a 3 pole 12v automotive relay capable of switching the same over more current that your fogs will ask for.