tail lights

[704x4]

i've been having problems with my lights in the rear of the truck(tail, brake, turn, reverse, and license plate). it has four wires running to the back of the truck they are spliced about five times, have dead ends, and are very brittle. so i went out and bought some heavy duty trailer wiring to replace the old. i normally have a pretty good grasp on wiring and how it works, but i have never done this before and i need to know what each wire does, where to slice it, which wires to split, and if the ground is seperate and to the bodyor frame. any diagrams or discusion will be greatly appreciated. thanks ahead of time.