The number plate bulb in your 2001 Ford Focus is likely required by law in your state. The light illuminates your plate number and tags so law enforcement officers and other drivers can clearly make out the information in the dark. Like all light bulbs, the Focus's number plate light has a limited lifespan and will need to be replaced at some point. The Focus uses a C-5W light bulb, which should be readily available at any auto parts store.
Open the boot on your Focus. The number plate bulb is mounted just above the number plate.
Use a flat head screwdriver to carefully pry the right side of the bulb assembly out of the bulb housing in the boot.
Slide the screwdriver toward the left side of the bulb, prying up as you go. This will work the bulb assembly completely out of the housing.
Pop the burnt-out bulb out of the copper contacts holding it into the assembly. Snap a new bulb into the assembly.
Reinstall the bulb assembly into the housing by pushing the left side into the housing and then working toward the right side. The assembly should slide right into the housing.
Close your boot and turn on your lights to test out your new bulb.
Replace your number plate bulb as soon as it burns out, or you risk getting a ticket.