Fog light bulbs should be checked every six months to make sure that they are functioning properly. The fog lights on most vehicles sit directly underneath the headlight bulbs. The ballast and wiring work just light the headlights, so changing them takes about five minutes. No special tools are required in most cases, and new fog light bulbs can be purchased from any auto parts store. The part number you will need for purchase of a replacement bulb will be stamped on the bottom of the old fog light (on the metal or plastic base).
Open the hood and reach down underneath the headlights to unplug the electrical plug from the fog light assembly. Squeeze the top of the release tab and pull the connection off the bulb's ballast.
Turn the fog light counterclockwise to remove it from the fog light housing.
Install the new fog light bulb. Then, turn it clockwise to lock it into the fog light housing. The part number you'll need for the replacement fog light can normally be found on the base of the old bulb. All you need to do is take the old bulb to any auto parts store and ask for a replacement bulb with the same bulb part number.
Connect the electrical plug to the back of the fog light ballast.
Verify that the fog lights are working by turning them on.
For specific information about changing your fog light bulbs, consult the particular vehicle's manual (see Resources).
Never touch the bulb with your fingers. Doing so will cause the bulb to explode inside the lamp assembly once it reaches normal operating temperature.