Trailer electric brakes do not have neg/pos specific wires. There are two wires that come from the magnet and one connects to the trailers pos and the other connects to the trailers ground, but it does not matter which wire is which from the magnets point of view. Just as long as one is pos and one is neg, that's all it cares about.
You should pull the drum and inspect the wires inside the assembly.. When my trailer was only a year old, I did my first wheel bearing service and found 2 of my wheels had cut brake wires inside the assembly. I spliced in longer wires and routed them so they would not get cut again.
When I replace the entire brake backing plates 2 years ago (10 year old trailer now) I just cut the 2 wires going to each wheel and when I put the new ones on, just spliced the 2 wires back without knowing (or caring) which wire was which.... The brakes work great! :)
Good luck!
Mitch