First, welcome to the forum.
Secondly, the polarity doesn't matter on the connection between the trailer wiring and the magnet.
Generally the brake wiring runs down one side of the frame, and drops to each wheel on that side where it connects to that magnet and to a pair of wires running inside the axle to the magnet on the other side. All the brake magnets are wired in parallel.
Also, the wire used on trailer brakes often has 2 layers of insulation on each wire, the inner layer being kind of translucent. If you didn't know it was there, you could miss that second layer and think you had the wire stripped when it wasn't.
Lastly, some trailers use those clip on wire taps, which I would avoid. Crimp connectors are stronger, but solder and heat shrink tubing would be best.
Dodge Ram dually laramie 4x4 w/jake, B&W, Brakesmart.
Kit Patio Hauler 394F toyhauler 5th wheel
Camo 680 Rincon, Green 500 Foreman, Blk twincam Roadking