Assuming you are talking about the taillight and not the entire tail assembly, here are instructions for removing a Ford Fiesta taillight. Tools you will need: Phillips head screwdriver, flat head screwdriver, ratchet and socket set.
1. Open your trunk and locate the two bolts holding the taillight in place. These are usually located at the top of the taillight.
2. Use a ratchet and socket to remove the bolts. Be careful not to drop them as they can be difficult to find if they fall into the trunk.
3. Once the bolts are removed, gently pull on the taillight to release it from its housing. You may need to use a flat head screwdriver to pry it loose if it is stuck in place.
4. Disconnect the electrical connector from the back of the taillight by pressing down on the tab and pulling it apart. If you have trouble disconnecting it, consult your owner's manual for more detailed instructions specific to your vehicle model year.
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