My ford focus 51 plate brake light not working, l change the bulb and fuse is ok, still not working.

check the relays please

If bulbs are definetly ok you can look behind trim on the left side in the boot, which covers the space between the boot floor and the window, you are looking for a green/red wire in a connector. this is the power cable which supplies both rear side mounted brake lights. You should check for voltage at this wire when pedal depressed. If no voltage you would need to trace this back to where the wire is joined in the loom as it feeds off the wire which supplies the high mounted brake light and you have stated that this works. Would still double check bulbs first though if you did'nt replace them. Hope this helps.

the switch is in fact the earth when you select reverse it gronds the wire, so to test take lead off and earth it, there will be a contant live to the bulb with the ignition on, if when earthed the light works the fault is with the switch, which could be an adjustment problem, or a defective switch. if does not work when earthed check the live to the bulb then check fuse

bulbs and fuses

Check the brake switch, located on the break pedal. Connect the 2 wires together and if the break lights work, a new switch is required.(if only 1 wire connect to frame/chassis)

The power goes through the turnsignal switch befor it gets to the tailights.

1 - are you SURE theyre the right bulbs! - if theyre too bright they can blow the fuse, if theyre the wrong type of bulb they will blow the fuse.. best get the bulbs direct from ford, they are always in stock, so are the proper fuses.
2 - theres a center brakelight isnt there? check this bulb too for proper wattage and size/shape
Other than that maybe the wiring in the boot is damaged and shorting?? -----
Leave feedback good or bad.

You  probably have a frayed wire touching something metal (short to ground) check your wiring, check your grounds.

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