Back up lights on a trailer are fairly useless.
First anytime you are backing you should have a spotter out.
Second, the illuminated area will be behind the trailer where you cant see.
Third...if you need to back into a particular place at night...say a very dark site or up your driveway...take your Maglight flashlight and place it on the ground where you want the wheels to be...then back down the light beam.
This is much more practical and useful than back up lights and it's the trick pro drivers (truckers) use to get into dark inside docks from the bright outside of daylight.