Adding extra "R" lights (like auto clear fog lights pointing backwards) on the rear of the Tow Vehicle works great when NOT towing a trailer. When towing a trailer, these lights often hit the front of the trailer and reflect forward (into the driver's eyes). If wondering, I installed extra rear lights on my previous Safari van. These lights worked great (when in solo mode) but hated them when trying to backup an attached trailer (because of their reflection off the front of the TT).
For pictures of their install, surf:
Knowing what I know today, I would simply install the same "low profile" clear lens auto fog lights. Install them under the rear bumper of Trailer, wire to the Trailer's 12V battery (using 12 guage + inline fuse) and install a manual off/on switch on its "+" wire. Switch would probably be installed in the TT's tonque area. When I want extra lighting at the rear of my trailer, I simply flip the manual switch. And if I wanted to backup in the dark with NO extra lights, I'd leave this manual switch in its off position. And to eliminate the risk of these extra rear lights coming on during long distance drives, I simply remove its inline fuse. And best of all, this custom configuration (using self or local grounding lights and only 1 x 12 guage "+" wire to TT's onboard battery) stays with the Trailer - regardless of which Tow Vehicle is pulling it. IMO, this is "clean and simple" install and should be "factory build" on every trailer.
Note: Some driver's may feel a manual switch is a pain but since one must always perform a circle check around their trailer before backing up, they are at their trailer's tonque area anyway. During circle check, they simply flip the manual switch to ON. And when disconnecting the TV (from the TT), simply turn this manual switch to OFF.
Hope this helps in your research....