You will need a pry tool..
On the Fiesta - any base model - it uses "plastic rivet" clip - wide-head to hold the headliner (and in this case
your 3rd brake light) to the interior roof. That big wide RED (or any color of your choosing-pick by outline) ones are the best. Their wide surface area lets you apply some gentle force and multiply it over a larger surface area so the trim and felt doesn't look funky when it's reassembled.
Plastic Rivet? They are made of metal and clip onto the Plastic tabs on the pieces and fit the slots - you will have to pry these out carefully so you don't break the tabs or bend the rivets to pieces and shred your work.
I Strongly advice using PPE to protect yourself from getting cuts and bleeding their design can cause. By their shards and sharp edges they can form once removed. The metal spikes - they can deform and can be considered "SHARPS" (READ
uncture - SEE: Needles) - you may need to order more newer ones to replace the older ones - if they are too deformed - just discard them - properly and carefully.
So you will need a pry tool; trowel or putty knife to help release the rivets carefully - you rock the tool under the plastic trim and the headliner - to help release the rivets from their threaded (or slotted holes shown below) holes - work one side to pop its rivet - then work the other side to pop the other clip-rivet.
To remove the lamp itself - two tabs one either end
are pried to release the lamp and lens TOGETHER
If you have a Hatchback - your 3rd brake light is outside on the spoiler.
To hold the interior headliner - they are simple plastic pop-rivets no metal is used,
They fit and go into those TORX/Hex Key holes just aside the outer seal gasket.
IF things get really bad, you can remove the entire assembly from the headliner.
(
That is if the headliner is separated or torn and needs replacement)
then the electrical connector is a press clip that can
then be pulled off to free the lamp and lens as one unit.
It's best to buy at least 6 bulbs (8 ~10 come in a case keep rest for spares) and replace
ALL bulbs
Reverse procedure - re-clip red lens and reconnect electrical connector and reinstall...
CAUTION: best to do a TEST to make sure all those lamps work, else you may need to
re-form; pry or clean the contacts on the lamp assembly to help re-establish the light so it works.
