Kevs57,
If you don't have turn signals, it pretty straightforward. Two wires to each light assembly. One wire for taillights...small filament, and one wire for larger, brighter filament which is brake light. These wires probably go to one light and are spliced there to go across the bed to the other light.
The 1 minute auto electric course tells you the power comes out of the battery, through the wire to the light. It goes through the light and does 'work'...makes the filament glow, then it must have a path back to the battery. This 'back to the battery' path is the entire frame and structure of the truck. You can see that this 'huge wire' is important. Everything must be bolted and connected to allow the power to reach the negative side of the battery.
Stuart
