Thinking about it that is kind of a cool idea.
Do they come on with just the parking lights?
Or do the headlights have to be on?
I have this little theory.
I see a lot of '97 and up XJ's with rusted out factory fogs.
I think some of that is lack use. Condensation and whatnot building up in there.
So I run mine, especially after it rains, in the hope that the heat from the bulb will dry up any condensation in there.
Maybe they won't rust out as quick. LOL.
My 914 fogs have their own switch, but either the parking lamps or low beams must also be on, regardless if fog lamp switch is on, for the fog lamps to illuminate.
High beam on, or parking lamps off AND low beam off kills the fogs.
An interesting thing on my 914 is if parking lamps are turned on, first on detent in switch, the parking lights go on, headlamps stay retracted. pull switch to second on detent, and headlamps raise and turn on, then move switch to first on detent parking lights are on and headlamps although no longer illuminating, stay raised. Headlights lower only when switch is in off position. weird german engineering.
Funny thing is, before the XJ, I worked on making my ride lower, stiffer, low profile tires etc.. handled like on rails when on a paved road. Had only a few inches clearance. Now with the XJ it is bigger tires, and lift, handling be darned for the paved road!
the added plus with the XJ is I dont need to jack it up to change the oil, although I do need to stand on top of the tire to reach the filter. I had to lift the 914 to adjust the valves even. Now my floor jack is nearly fully extended before starting lift of the XJ, where as on the 914, fully contracted it barely made it under any jack point.
I digress.
for rusting factory fogs, I have seen that at the wrecker, have not found not rusted ones yet, Suggest if yours is not rusted out yet, get the housing bead blasted or such and then coat with some good epoxy type paint inside and out. white on the inside, or better still, after paint is fully cured, line the reflector with aluminum foil ducting tape. this tape also works great to line tarnished tail lamp reflectors. some factory reflectors are simply paint, and dont hold up well, the tape trick was done on my XJ tail lamps, which were not very reflective, the improvement was amazing.
anyways with clean metal painted with good paint you should be set for another 20 plus years of fog lights. also consider sealing them well after painting, a bead of silicone rubber, or simply add a drain hole, but keep in mind drain holes can let in water and dirt, so you may end up taking them apart frequent like for cleaning. sealing them might be best option once painted drain holes can let in lb busting cold water if you drive thru high water, in which case turn the lamps off and allow to cool, the thermal shock hot to cold from a flooded housing can break the bulb. little one way rubber valves can in installed for drains. They make rubber valve like this for lamps housings . shine on!