Chuck - Do you have an rv type plug (7 pin) - if so - there is a hot wire in that that you can raw power off of. If not - One way that I have seen it done is get a pair of quick connectors that run from the battery to the back of your truck. Then you can plug up to that connector with one mounted to the trailer. If that doesn't make sense - I will try to get you some pics. I know Copacabana has pics of his dump trailer hooked up like that.
Hope that helps some!