The halogen headlight bulbs in your Grand Marquis should typically last well over 50,000 miles with standard use. A damaged or defective headlight bulb commonly attributes to water damage or a problem with the bulbs wiring connection. Rocks and various other types of road debris often hit and damage the headlight lens while you're traveling. Even the smallest crack in the lens will allow water to penetrate it and cause bulb failure as a result. Installing new headlight bulbs in your Grand Marquis is a minor task that you can do right at home. With a few minor tools, you should be able to complete the task within 25 to 30 minutes.
Turn on the power to your Grand Marquis's headlights, and proceed to the front of the car. Identify which of your two headlights has burned out then turn the power for the headlights back off.
Open the hood of the Grand Marquis and remove the negative battery cable with a socket wrench. Beside each of the cables metal terminals, you should see a small inscription on the battery's case. The one with a "-" sign next to the post it is attached to is the negative cable. The positive cable will be attached to the post with a "+" sign on it.
Stand directly in front of the affected headlight bulb on your Grand Marquis. Look down into the engine compartment at the back of the headlight assembly. You should see the headlight bulb attached to the rear of the lens. If your Grand Marquis has a black piece of trim covering the rear of the headlight assembly, remove the small bolts holding it in place with your socket wrench. Lift the trim out of place, and set it to the side along with its bolts.
Slide the tip of a slotted screwdriver under the safety clip on the headlight bulb. Pry the safety clip upward off the retaining tab on the bulb. Grasp the plug at the base of the headlight bulb and pull it away from its connection to the defective bulb. You have to keep the safety clip raised while doing this, and don’t pull the plug by its wires or more unwanted damage will occur. Note the direction that the headlight bulb is pointed within the lens. You must replace it exactly as you see it or the headlight's aiming will be off dramatically.
Grasp the back of the headlight bulb and rotate it one-eighth of a turn counterclockwise to unlock it from the lens. Pull the bulb carefully away from the lens on your Grand Marquis, and set it to the side.
Put on a clean pair of latex gloves.
Insert your Grand Marquis' new headlight bulb carefully back through the opening at the rear of the headlight lens. Before you lock the bulb back into place, make sure that it is in the same position within the lens as the old bulb was.
Lock the new headlight bulb into the lens by rotating it one-eighth of a turn clockwise. Plug the headlight's power connector back up to it. You do not have to lift the safety clip again, but you should hear a small click letting you know that the safety clip has fully engaged the retaining tab on your new headlight bulb.
Reinstall the piece of trim that concealed the back of your Grand Marquis' headlight assembly if you had to remove it to access the bulb. Tighten the trim's securing bolts with your socket wrench until snug. If you plan to replace the other headlight bulb, use this entire process to complete the task. If not, connect and secure your Grand Marquis' negative cable back to the battery with your socket wrench. Shut the hood on your Grand Marquis, and turn on the power to the headlights once more to check the repair.
