I figured I'd eventually mount some work lights on the neck of my gooseneck trailer so they're up and out of the way. I also want to add some backup lights to it too. It's a long way to the ass end of a 26 foot gooseneck! I think something flood light-ish with a wide pattern would probably work best for a backup light. I've thought about mounting some under the trailer so it would illuminate a good bit of area, not be blinding, be out of the way, and hopefully let me see more of the tail end of the trailer. I'd have to do some tests on the placement to see what would work best.
If I had a bumper pull, I'd 100% have work lights up top busted or broken off. I can't really think of anywhere to put them where they'd be both protected AND throw usable or helpful light. Maybe on the steps fore and aft of the fenders, kinda 45'd inward, if it has fenders/steps. I might would grab some magnetically mounted lights and move them around as needed or just wear a headband light.
All that said...I vehemently try to avoid, if at all possible, loading or unloading in the dark. Truthfully, I'm not even a huge fan of towing in the dark, but if such is the case, I always make sure I'm loaded well beforehand and will usually wait until the next day to unload. That's just me though!
For sake of conversation, what kinda trailer are you working with here? Post some pics, that'll definitely help with ideas. Everybody already hit the wiring aspect dead on, just gotta figure out what to use and where to put them.