From time to time you'll find one of your headlamps has a burned out bulb. It's a low-cost replacment part and only takes a few minutes to replace.
The headlamp uses several small 12-volt halogen bulbs of type "H7". There are 3 bulbs in each headlamp: one for the low beam, one for the high beam, and one for the fog light. It surprised me that the same kind of bulb provides the normal beam and the much brighter high beam; the light is reflected differently inside the headlamp to make this happen. The bulbs operate independently and any one of them can burn out on its own; since you use your low beam the most, it's most likely to be the culprit if you notice that a headlight is out. Oddly, even when the low beam is burned out, the headlamp is not completely dark (I think there's a dimly-lit standby lamp that is always on when the headlight switch is on).
You can get an H7 halogen bulb from any car parts store. It should be less than $10.
H7 halogen bulb
You'll need a tool from your spare tire tool kit. The kit is located behind your spare tire. Loosen the spare tire wing nut so you can pull the tool kit out. The tool you need is a hex socket with a screwdriver-like handle that folds out.
hex socket tool
(click photos to view full-size image)
First you'll remove the entire headlamp unit. It's one big piece containing the three bulbs and all of the reflection chambers.
1. Pull back the trunk liner carpet. To do this, undo the plastic nut that holds the carpet to the side. Unscrew the nut while pulling, and eventually it will come off the threaded post.
2. Remove the round rubber plug that covers the locking lever.
3. Insert the hex socket into the hole that was plugged by the round rubber plug until it grabs the hex head inside. Give it a 1/4 turn towards the back of the car (that's counter-clockwise here at the left headlamp). You will hear a solid click or snap and the headlamp will pop out about a 1/4 inch or so. What's happening is you are turning the locking lever, releasing the headlamp and actually pushing it out of its socket slightly. Leave the lever in this position until you are ready to lock the headlamp in place at the end.
4. Pull the headlamp unit straight out of the housing in the fender. You may have to wriggle it side-to-side as you do this, but if it's stuck you probably did not unlock the locking lever completely.
You can move the headlamp unit to a convenient place to swap out the bulb.
1. Looking at the rear of the headlamp unit, you'll see a large oval-shaped cover, with the electrical connector on the left, and a smaller circular cover on the right farther toward the front of the unit. The large cover provides access to the low and high beam lamps; the small cover is for the fog lamp. Open the appropriate cover. I'll assuming you're replacing the high or low beam in the photos from here on, but the same steps apply to the fog lamp.
2. The low beam is the top lamp; the high beam is the bottom lamp. Pull the electrical connector off the lamp.
3. Unlatch the wire latch that secures the halogen bulb. Now the bulb is free and can be removed. Don't worry about the bulb orientationthere's only one way it goes in.
4. Check the bulb to see if you can see the burned-out filament. If you can't see it, maybe the other bulb is the one that is burned out. But you should already know based on whether your high or low beam is the one that has failed.
5. Put the new bulb in. (Warning: do not touch the bulb glass with your fingers, because the oil from your skin will cause the bulb to deteriorate rapidly. Handle the bulb by grabbing the electrical connector prongs.) Latch it down. Re-connect the electrical connector.
6. Close the cover, snapping or twisting it shut as applies.
1. Slide the headlamp unit back into the housing in the fender. You'll notice three guide tabs on the sides of the headlamp unit, one on each side toward the rear, and one on the outside closer to the front. These slide into tracks in the fender housing. You can start by guiding the rear tabs into the tracks. Because nothing is greased, you'll have to wriggle the unit a little to get it to slide straight in. When the unit is about halfway in you can guide the front tab into the outside track and then slide the unit the rest of the way in. You should slide it in as far as it goes, which should leave about a 1/4 inch gap between the rubber gasket around the unit and the car's fender.
2. Go back to hex socket tool behind the trunk liner carpet. Give it a 1/4 turn towards the front of the car (that's clockwise here at the left headlamp). You will hear a solid click or snap and the headlamp will be pulled in flush against the fender to its final position.
Warning: If you do not hear the unit snap into position, it is probably not locked in place, in which case the headlamp unit may fly out under hard braking!
3. Replace the rubber plug covering the locking lever hole.
4. Reattach the trunk liner carpet by pushing and/or turning the plastic nut. Tuck the carpet under the rubber gasket that surrounds the trunk.
5. Test it and see if the lights all work!