Well, can you determine if the "Earth" aka Ground - wire - is that bad, or are you having problems with the connector?
In a Fiesta - the Brake Light switch is on a bracket - and next to it, is also a secondary switch used for the PCM / Body Control Module - just in front of the brake and just behind the kickpanel.
The Brake light switch is the Dark colored one, while the PCM/BCM one is bright blue body. Turn them 1/4 turn counter clockwise to remove - it will pop out thru the lock hole (it's keyed to fit one way) to install you reverse the process, just insert plunger thru the hole then gently center the "Tangs" to allow the switch body shaft to start to seat, then pull switch body towards the plunger hole - makes the spring the holds that tang in place, move so you can then twist clockwise - to let the notches line up and lock the switch into position.
So if the connectors are seated - then you'll have to remove the seat to floor bolts and try to lift up the carpet - removing floor pan to carpet the trim panels to do this - what you will find below it, right by and just under (that long brace bar-rise the seat bolts to) is a big lug on a bolt with a lot of wires tied - soldered to it from the harness - there are two of these - one under the Driver side seat, and the passenger side seat.
This is where it gets ugly - you will have either commit to a shop, or try to fix this - but either way - this is how you inspect the ground bonding harness