Ollo Ollo,
Can you describe the physical and electrical setup of the box itself? For instance:
Is it mounted on your rack? How attached? What kind of connector/plug are you using at the box? Do you unplug the lights to plug in the charger? What kind of connectors have you used for making the other electical connections - Wires twisted together and taped? Spade connectors, aka, auto disconnects? Some other type of plug? Trailer plugs? What kind of wire - Zip cord? Lamp cord? Trailer wire?

Right now I have a combination of the above connections. I am pretty well satisfied with my setup overall, but I am always looking to improve the details.