Well I always start with the simple things. See if there's bad ground wire in the back because if it's a bad ground wire it won't get any power. And of course check all the fuses to see if there's any problem with a blown fuse. And pray it's simple like that because realize ultimately the body control module on that vehicle controls all the lighting system and they are really expensive to replace and reprogram if they go bad. Good luck

F150 Tail Light Troubleshooting‌

‌Fault phenomenon‌: The tail lights (including turn signals and brake lights) of the 2009 F150 Platinum 5.4 stopped working.

‌Possible causes‌:

‌Circuit problem‌: The problem occurred one week after replacing the alternator, which may be related to the circuit connection.
‌Bulb or lamp group failure‌: A damaged bulb or a circuit failure inside the lamp group may cause the tail lights to not light up.
‌Fuse or relay failure‌: A blown fuse or relay failure may also cause the tail lights to fail.

‌Troubleshooting steps‌:

Check whether the tail light bulb is intact and replace it if damaged.

Check the circuit connection, especially the part related to the alternator.

Check the fuse and relay to make sure they are in normal working condition.

If the above steps cannot solve the problem, it is recommended to seek help from a professional maintenance personnel for further diagnosis‌