Even many Ford mechanics agree that the tail light removal in a Ford Focus is somewhat cumbersome because of the care which must be taken not to break the plastic pegs holding the tail light assembly in place. Be certain that you purchase the correct tail light assembly--there is a different one for the driver's side and the passenger's side--and make certain that if you need a new bulb that a bulb is included, as bulbs must be purchased separately many times.
Open the boot and locate the panel just behind the tail light that you wish to fix.
Using a Phillips screwdriver, remove the two screws holding the panel in place. Set the screws aside in a safe place. The panel will pull straight off once the screws are removed.
Find the two plastic pins and the spring assembly that is holding the tail light assembly in place. To find the pins, stand behind the car with the tail light directly in front of you. Bend over and look into the hole where the panel was, and gently pull the tail light directly toward the rear passenger seat of the car. As you to so, you will notice the two plastic pins on the left side of the assembly if you are working with the driver's side tail light and on your right if you are working with the passenger side tail light.
Pull the right side of the driver's side tail light outward slowly and gently twist it until it slips off the plastic pegs. Pull out on the left side of the passenger's side tail light and gently twist until the tail light slips off of the pins. A spring bracket will pull out of its hole regardless of whether you are working on the passenger's or driver's tail light.
Hold the tail light assembly in your hand with the wires still attached. Replace any broken plastic parts at this point. To replace the tail light bulb simply push the bulb in slightly and twist it to your left. Release the pressure and the bulb will pop out. Push a new bulb into place and while pushing down slightly, twist the bulb to the right until it locks into place.
Replace the tail light assembly by holding the assembly at a slight angle so that you can insert it onto the two plastic pins. Push in gently and twist slightly until the spring clip slips back into its hole. Push firmly to snap everything back into place. Replace the panel and replace the two screws to complete the job.
Do not force the assembly out or back in. Twist and pull gently until it slips off of the plastic pins. If you break one or both of the pins, your tail light assembly will not remain in place firmly.