How do you take off the light cover on a 2007 ford focus to put a new low beam light on

owners manual fuses start page 221
https://cdn.dealereprocess.net/cdn/servicemanuals/ford/2014-focus.pdf
lights starts page 90
change light bulbs starts page 246

if just one bulb quit working might be the bulb, connection, wiring or ground wire.
Some cars may have a fuse for different light, or multiple lights. From what I see you do not have a fuse for one low beam light.
Some cars have relays for example high beam lights I don't see a relay for one low beam light.
I think you have a bad bulb or light socket or wiring.
You can try putting the drivers side bulb in the passenger side see if it works that should tell you if it is the bulb.

sounds like you replaced them because of this issue.that leaves the dimmer switch likely culprit

un plug one of the light connectors and check for power and verify ground with a 12v test light tool (available for cheap at the part store.) if there is 12v present, and a ground when switched to low beam, remove the bulb, inspect the filament. you will likely find it just has two burnt light bulbs

These trucks are known for bad multifuction switches.
the switch that i talk of is the signal switch , the handle is the switch, it bolts to side of steering column . this switch controls the signals and the headlights and high /low beams. the other thing is the headlight switch connector, the connector some times goes bad and starts to melt or just comes out of the connector. you will need to remove the cover to steering column, the screws are located under the bottom cover in holes. remove cover and check connector and then test for power if you have power going into switch but not coming out then switch is bad.

the last thing is to make sure the bulbs are good , make sure the headlight bulbs are good , and also the driving light bulbs, test for power at the connectors to those lights to.
You will need to do a little testing to find the problem.

more than likely a fuse or a relay is bad. locate the fuse for the headlights (owners manual) and check if it is bad, if the fuse is good, find the relay and give it a flick or two and try the headlights then, if they work, its time for a new relay

Bulbs are never covered under warranty because they are a wear item (like tires). The factory might do a courtesy repair if the car was near new.
Order the bulbs online and install them yourself. I believe the bulb is D2s, you can find them online for $30-$60.

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Turn the engine off, open the hood, and disconnect the negative battery cable (--) with your pliers.

Locate the power distribution box against the firewall on the driver's side of the engine compartment. Flip up the latch and remove the plastic cover.

Determine which headlight fuse you want to replace. The 2001 Ford Focus uses four headlight fuses--one for each headlight. Fuse number 16 is for the passenger side low beam; fuse number 17 is for the driver's side low beam. Fuse number 26 is for the passenger side high beam; fuse number 27 is for the driver's side high beam.

Grasp the fuse with your fingers and pull it straight out of its mount. Insert the new fuse in its place. Replace the plastic cover and reconnect the negative battery cable.

just replace the dimmer switch under the column its wore out on the low beam side

Should be fairly easy. Make sure to disconnect the positive battery cable. Remove the cover from around the column behind the steering wheel. Should take a star driver to remove the screws holding the cover together. Once the screws are removed pop the cover apart and you will see the multifunction switch on the column, should have two to three screws holding it to the column remove the screws and it should come out pretty easy replace with new one and put back together.

The light bulbs may need to be replaced, they burnt out on the low beam only.