In my 2021 5.0 ETI made the final connection to the taillights on both sides with a WAGO connector. On one side it was barely visible under the edge of the flexible wall material ending under the dinette bench, the other a bit higher under that wall covering but easily exposed by peeling up the material (again, all below the bench seat). The WAGO connectors lend to easily lifting the lever for each wire and adding another for aux lighting as appropriate - no cutting or alteration of the trailer's OE wiring required.
Here's a page showing a typical WAGO connector with videos showing how they work.
Your aux light wires will be very small, you can easily drill a small hole to pass them through the bottom of the shell under the dinette and use your preferred sealant (Proflex, etc) inside and out to make that penetration watertight.
As long as you use commonly available LED trailer-type fixtures for your aux lights there will be no issue with the slight additional load in parallel on the existing circuit.
Just one option for your consideration, IMO much easier than running and securing wires from the front of the trailer and perfectly suitable to the need, have fun!
I did not add aux taillights to my trailer but did replace the OE taillight fixtures with larger LED taillight arrays, making the connection at those WAGO connectors (and adding backup lights to aid rear trailer-cam visibility in low light when backing)
Add rear lights to hitch box?
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