I have just installed an LED light assembly for my Ender 3 V3 SE printer. Compared to the instructional video, I decided to make some small improvements. Namely, I sawed a small hole (marked on the photo) for the cable passage and ran this cable through the right side of the gantry. This makes the cable going all the way inside the unit more resistant to damage. It also looks better (OK, it’s almost invisible) 
It is also worth paying attention to the connection of the switch and ensuring that the cable passes inside so that it can be cut through the sheet metal. The rest as in the instructions.
BTW. it is not at all necessary to unscrew the gantry from the bottom. Just run the cable through as much as possible and pull the ends out with tweezers.
Perhaps it will be of use to someone…
Greetings,
Marek.