On my Neon R/T with Fogs, the mod was to splice a ignition switched power wire to the power side of the wire to the fog light switch. Should work with an XJ fogs, provided you can find an ignition switched power wire with enough extra capacity to safely splice into.
The fogs only went on with the low beams, off with the head lamps off and any time the high beams were on. The Rear Defrost switch is right next to the Fog Lamp Switch, I just spliced the power sources to both switches together and now the fogs will turn on anytime the ignition is on. Yes, I've checked, defog can supply enough current for the fogs, I've run both at the same time, the wires don't get warm nor do the fuses blow.