How to replace fog light

"Not working" can mean a dead bulb or a dead circuit, and the fix is very different, so diagnose first — it takes five minutes and saves you buying a bulb you don't need.

First, figure out which side of the problem you're on:

1. One fog light out, the other fine? It's almost certainly the bulb or that one socket. Swap the bulbs side to side — if the dead side starts working with the good bulb, you just need a new bulb. If the same side stays dead with a known-good bulb, the fault is in that side's socket or wiring.
2. Both fog lights out at once? Both bulbs rarely die together. Check the fog light fuse in the fuse box first (your owner's manual shows which one), then the fog light relay — swapping it with an identical relay from another circuit is a quick test. If the fuse is blown, replace it once; if the new one blows immediately, there's a short in the wiring and you need to find it before feeding it more fuses.
3. Check the switch and the ground. If the fuse and relay are good, turn the fog lights on and check whether the switch is actually sending power. A corroded ground wire at the front of the truck will also kill both fog lights — follow the ground wire from the fog light harness to where it bolts to the body, clean it to bare metal, and tighten it.

Replacing the bulb (once you've confirmed the bulb is the problem):

On the 1999–2004 Xterra the fog light is reached from behind the bumper — you don't need to pull the whole bumper cover off. Turn the steering wheel away from the side you're working on, reach in behind the bumper (or pull the small splash shield back), and find the back of the fog light housing. Unplug the electrical connector, twist the bulb holder counterclockwise, and pull it out. A replacement fog light bulb commonly fits your Xterra, but always confirm against your vehicle's exact trim and engine and the product's fitment data before buying — and match the bulb number printed on the old bulb.

Two things that matter: don't touch the glass of the new bulb with bare fingers — skin oil creates a hot spot on the glass and shortens bulb life; handle it by the base or with a clean cloth. And if the socket shows green corrosion or melted plastic around the terminals, replace the socket too — a bad socket will kill the new bulb in short order.

If the whole housing is cracked or full of water, the bulb isn't your problem — the housing is. A replacement fog light assembly commonly fits your Xterra, again confirming fitment first. And if the fuse, relay, bulbs, and grounds all check out and they still won't light, that's a wiring or switch job — if unsure, consult a professional mechanic.

Disclosure: I used an AI tool to help draft this answer.