This page of the Far Cry 6 game guide contains a walkthrough of the "Sweet Fifteen" Treasure Hunt available in El Este.
Subregion: Sierra Perdida.
District: Nueva Vida.
The mission is initiated by reading the note located in front of the building. Upon reading it, the door will open. Once inside, you need to complete a task.
The task is to use the 4 switches. Start with the one on the right side of the warehouse. Just destroy the planks (it's not worth using incendiary weapons) and then make your way to the end of the corridor.
Continuing up, head just under the roof of the warehouse, where you will reach a room containing the next switch. Use it and then return to the middle of the building.
Time for the other side, where you can see electricity sparks. There's no need to be worried. Just walk on the furniture to avoid stepping on damaged cables. When you reach the button, you will be able to turn off the electricity, stopping it from doing damage.
Turn to the right to reach the next switch you need to use.
The last switch is located in a locked room. You can open it by shooting the padlock.
Finally, you need to walk out into the middle of the warehouse, flick the final switch, and exit the building. This will complete the mission and you will be able to ride off on your new mount.