I put all new lights (LED) on my
trailer, but for highway and bad weather driving, I added a light bar to the outboard, as it's the furthest thing back, and a rather long way behind the normal (for North America) trailer lights.
You can't even see the trailer lights under the boat when sitting high and/or close, so I wanted lights right in the eyeballs of whomever thinks climbing up my ass will make me go faster.
It's also a requirement in most states to have a light on, or close to, the end of a load hanging off the back of a trailer at night, but boats seem to get a little slack (just from what I've read here) due to the nature of the beast.
It's made of scrap 2x4's, some scrap ply, Harbor Freight LED lights with side markers (super cheap Chinese imports), some bunk carpet so it doesn't scratch the motor, and a bungie. 4-conductor trailer wiring by the foot, and a plug. When I wired the trailer, I used a junction box at the winch post and hung a short pigtail with the car end (receptacle) that came with the lights, so I just plug the long wire in if I'm going to use the light bar.
Not too elegant, but quite functional. Others here have made much prettier light bars, usually mounted to the stern rail, but I wanted them aaaalllll the way back.