I was fooling around with my 90 limited today (coolant flush & replacement) and while 'up front' I was scoping out my non-working fogs. They may or may not be OEM, but are tied into the OEM plugs. I 'think' if you have the OEM switch on the dash, at some point there was factory - or dealer installed - lights on there. I poked at the wires at the backs of the lamps and found voltage...so I know that it's just the lights/bulbs...the inside/reflectors of these are rusty as crap and I plan to replace with Hella 550s next payday.
I'd round up a test light or DMM and turn on the factory switch...it's next to the headlight switch...mine has the fog lamp sw on the far left and the rear defogger sw on the right (left side of column) Test for 12v at the plugs (outside/in front) if you find voltage, simply wire up a set of lamps & be happy! I'd keep them at 55w or less (like Hella's are) The OEM relay & circuitry should be OK with that little bit (-10 amps)
If you get no voltage out at the plugs, I'd then use the whole harness that (should) come with the lamps... and run the switch wires through to the dash somewhere. OR skip the switch altogether, and wire them off one of the headlamp power wires if you want them to kill when a certain beam is selected...powerful driving lights can be wired to kill when ya dim to low beam...no fumbling for another switch while blinding the oncoming Highway Patrolman
Note: I 'think' the factory fog switch is wired in so that your fogs will cut off when hi-beam is turned on...some arcane DOT reg, that. using the harness & switch that comes with the lights, wired directly to the battery will allow use of the lamps at any time, regardless of whether the key is on...this 'could' be handy, but could also kill your battery/melt the plastic covers if accidentaly turned on or left on unintentionally.
FWIW my 89 MJ had no OEM switch, but did have the harness inside the dash behind the trim...I don't think it had the leads out at the bumper.
HTH and I'll share more when I know more...I'm kinda new to the extra wiring on my 88 and 90 limiteds