Just rewiring should be easy and inexpensive. Wires are wires -- materials are cheap and anyone who understands how they work should be able to do it quickly. Just call up your local RV or trailer dealer and ask for a quote.
Usually brakes and lights run me a few hundred, including when the trailer guy found that one of our brakes on our new-to-us trailer had been put on upside down (??) and he had to change out a bunch of broken lights.
Call and ask - our repair guy is always pretty good at giving an estimate that is fairly accurate.
Actually, wires aren't simply wires. Cheap wire is just that-cheap wire. A lot of trailer wires for some time now has been copper coated steel, whether on a horse trailer, or a boat trailer. The copper coating gets comprimised for all sorts of reasons, and the steel rusts. Rusty steel is a poor conductor. Rusty wire on a rusty connection looses the ability to ground, and without a good ground, even the best connections everywhere else mean nothing.
Using crimp-on connectors just compounds the problems.