I don't know if it's best, but one reasonably decent way is to put the wires in a length of split loom tubing, and then attach it to the frame underneath with zip ties at intervals where possible. If you can thread it above the frame or otherwise through a more protected place, so much the better.
The split loom tubing helps protect the wires from the elements and from being crushed with the zip ties. One source of it at pretty reasonable prices is
All Electronics, although if that's all you get the shipping and handling will exceed its cost. (Incidentally, they also stock Delphi WeatherPack connector kits at pretty decent prices.)
