Assuming it is the same as the 1998 Mercury Tracer :
The fog light switch is on the turn signal lever , next to the
head light switch
* you probably need to have your head lights on low beam
The fog light switch on the 98 Mystique is on the headlight switch. When you turn on the headlights, pull the switch towards and the fog lights should come on. Here is a link to the owners manual for the 98 Mystique http://www.analogstereo.com/pdf/om/mystique_owners_manual_1998.pdf
The fog light switch is on the end of the turn signal switch. Just pull the end of the turn signal switch and the fog lights will come on. (Turn signal switch and fog light switch are one assembly)
What I do is put my headlights on ( low beam ) and pull the headlight control towards me to turn on the fog lights ( push the control in to turn off the fog lights )
pull out the light switch
On the turnsignal/light switch lever, on the same turn ring switch as rear fog light. If not you got change it to one with it.
Turn your low beam headlights on and pull the headlight knob out towards you to turn on your fog lights ( push the knob back in away from you to turn the fog lights off )
The fog light switch on my peugeot 106 is next too the hazard warning switch and the rear window heater switch below the stereo.
PULL the light switch out (must be in lowbeam setting for fog lights to work)
fog light relay is in the headlight switch durangos are known for this part going bad
pull out light switch
With main switch light just pull
To turn on the fog lights in your 2003 Mercury Mountaineer, first, ensure your vehicle is in the "On" or "Run" position. Locate the fog light switch, which is typically found on the headlight control knob or stalk. Turn the knob to the fog light icon or pull the stalk towards you (depending on your model). Make sure your regular headlights are on, as fog lights usually only operate when the headlights are activated.