I have towed for many years with a splitter, and not had any problems. If the wires are pulling out, then either they have not been screwed in tight enough, or more likely, the set up too short somewhere - perhaps the bungee is restricting cable flexibility too much?
Re wiring is always fiddly, but easy enough - just do one wire at a time, and maybe take a pic, if you don't have a wiring diagram to hand. Add a drop of Loctite threadlock to the screws before you tighten them, then they won't undo themselves - its available from motor factors and model shops.