If you have 4 brakes use 4 brakes. They are there for a reason.
Use the 10 gauge wire as stated before the length is what really determines the wire size.
Look on Amazon for the wire. I just rebuilt a 33' trailer for our ATVs and I bought everything through Amazon as it was considerably cheaper and two day shipping free.
As for the ones that aren't working start by running the right wire and checking connections and work from there. We just had to have 3 magnets replaced on our 2014 bunk house. They just stopped working.
With the wire connections try to use the shrink wrap to seal them up from the elements.
Check adjustment too while your under there.