Well, by suggestion, this isn't working.
So when you say "Flickering" I presumed you have a station
Listening problem.
The Condition can also be a station "
Viewing" problem, as in a flickering caused by the inability to view the APIM data on the screen due to a loose or faulty CAN-Bus link to the APIM from the Radio / Audio Module (your in-dash head unit for the radio and or CD/DVD an satellite presentation) that displays the data messages.
Are we getting close?
But, this may mean something else is the problem in a wiring harness and - if you can, take pictures so we can see your dashboard to help you further.
And, to help narrow this down - do you have the HD-unit?
Although from a Ford Focus, These are also available in Aftermarkets for Fiestas' - only seen one with one though...
We also have several Eco-sports in the fleet and they use similar setups...

The HD-units - The radio / APIM display is more like a tablet and about the size of one. IF that is what you have; the HD-unit decoder for the FM-stations - the rear antenna amplifier is still suspect. This is due to the level of sub-channel information that the HD units processes and displays on your cars' tablet-screen. If the system can't decode that information for the stations sub-channels - it flickers data in the display and it can even affect audio. It will - cut in and out leaving you with sometimes terrible sounding fuzzy audio-until the signal strength comes back to a point where it clears up from the noise and you have your "surround sound" audio back.
This may also indicate a loose ground or poor connection to a device (including the Fuel Pump or Rear-Brake light or even one of your SRS sensors) nearby or in the rear-(drivers side) passenger door that has come loose or it's connector is not fully seated.
The shop may have to go back in there and fix or re-dress the headers those connectors use and also fix any poor grounding issues in that area.
To test this... turn the Radio on to the AM-stations and listen for noise you'd find in-between stations and or weak signal ones and listen for noise to see if stronger stations also pick up this noise - if so - the car needs to get back to that shop for them to tighten down their work.