The 2007 Ford Focus uses a set of HB5 halogen bulbs in a composite headlight assembly to provide the forward light that you need for driving at night. The high beam and the low beam function use two different filaments within that one HB5 bulb. Replace your halogen headlights as a pair for best performance.
Open the hood of your Ford Focus and locate the headlight assembly. Locate the large cap on the rear of the headlight assembly and pull it off. Set the cap aside in a safe place for use later.
Disconnect the wiring harness connector from the rear of the headlight bulb by lifting the locking tab, being careful not to break it off. Pull the connector off the back of the bulb. Pull on the connector body only--not the wires--or damage will occur to the wires.
Turn the bulb retainer ring on the back of the assembly counterclockwise, then remove the ring and slide the bulb out of the headlight assembly. Discard the old bulb and insert a new HB5 bulb into the assembly. Do not handle the glass portion of the bulb with your hands. The oils from your hand will cause the bulb to overheat and shatter.
Install the headlight bulb locking ring over the back of the bulb and turn it clockwise until it locks in place. Install the wiring harness connector onto the rear of the bulb, pushing it on until the locking tab engages the pin on the body of the bulb.
Push the bulb cap onto the headlight assembly to keep the water and dirt out of the assembly. Move to the opposite headlight and repeat the process to change the second bulb.
Test the headlights by turning on the headlight switch and visually verifying that the headlights are working on both high and low beam.
Make sure the headlight switch is off and the headlights are cool before changing the bulbs in your Ford Focus.