Replace number plate bulb

This may help
Low Beam Headlight Bulb Size: 9003/HB2
High Beam Headlamp Light Bulb Size: 9003/HB2
Parking Light Bulb Size: 3157LL
Front Turn Signal Light Bulb Size: 3157LL
Rear Turn Signal Light Bulb Size: 3157LL
Tail Light Bulb Size: 3157LL
Stop Light Bulb Size: 3157LL
High Mount Stop Light Bulb Size: 2825
Fog/Driving Light Bulb Size: 899
License Plate Light Bulb Size: 6418
Back Up Light Bulb Size: 3156LL
Front Sidemarker Light Bulb Size: 194
Map Light Bulb Size: 3886X
Dome Light Bulb Size: 6411
google_protectAndRun("ads_core.google_render_ad", google_handleError, google_render_ad);

Sounds like something is connecting to the car body, I would inspect the bulb you changed make sure the metal clips or wires are not touching anything.

Slide the entire license plate lens assembly towards the driver's side using your thumb or a screwdriver. Once you have slid the lens as far as it'll go, take the screwdriver and pry on the lens gently and it should just pop right out.

bulbs and fuses

if you buy some of those "on tv only" lights that are bubble shaped and plastic, you can duct tape those onto your license plate area and click them on or off when needed.

Take a thin blade screw driver and gently pry between the rubber seal and the metal and pop the light out, replaced the bulb and pop the light back in. works fine.

Very easy, 2 or 3 minutes.
Remove radiator grille. 2 plastic fittings on top, quarter turn and lift out grille.
Remove complete headlight. Simply lift out 2 L shape retaining pins. Disconnect lead. Ease out complete headlamp unit.
Remove back of headlamp unit to access bulbs. Disconnect lead from bulb while bulb still held by clip. Then release clip to remove bulb.
Reverse process to fit.

It turned out that the bulbs in the headlights had gone simultaneously. Luckily my son is knowledgeable about cars. He came over and figured out the problem. I had to purchase new bulbs, then my son had to practically dismantle the front of the car to change the bulbs. It was a complicated operation just to replace bulbs, and something I would not have been able to tackle myself. If it had not been for my son. I would have had to spend a fortune at a garage for a mechanic to replace the bulbs for me. What should have been a relatively simple task has become a mammoth operation. In years gone by it would have been a simple task that I could have performed myself. Why do manufacturers design cars so that it is so complicated just to replace the bulbs. There are so many things that have developed faults on this Ford Focus, I will never have another one.

only one light not working indicates a bad circuit at the plug on the back of the light housing or in the wiring between the light and the dash swith

am not familiar with the early year Focus'. Look above the license plate, if the lens for the bulb has two screws, you need to remove them to access the bulb. On other models, open the trunk and look down where the license plate is, you should be able to see and access the bulb from there.