As long as all 4 brake assemblies are connected, you will never see 12 volts at the magnets, each of them are a designed short to ground and, will drop the voltage. Every splice in the system adds resistance and will drop voltage. If the splice is not a solid connection and water tight, they will corrode, add resistance, and drop the voltage. The cross over wire are usually inside the axle tube, where the jump around, rub, and chafe, which will short to ground and drop voltage.
I can go on, but I digress.
To improve performance, each magnet should have equal wire length to the source. The way they wire them, using under sized wire, is to run one lead down one side or the other, splice at the first axle, then to the second axle, then splice each axle to the other side. Resulting in the farthest wheel from the source will be the weakest braking wheel while the first in line closest to the source being the strongest.
The best way to wire them for best performance is to run 2 wires, one down each frame rail, then splice them at the center of suspension to each wheel. That way you will have the same wire length to each wheel.
And while your at it, use 10ga wire down the frame rails and splice 12ga to each wheel.
When all done and said, and got tired of not having comfortable feeling braking, I replaced the whole system with disc brakes and have never looked back.