So i'm having a moment here. I want to wire a relay to send power to the back of the truck can't the description is a bit vague. Relay No, i don't need that much capacity but i figure why not. ...
You can find an ignition controlled fuse and tap it to control the relay. Just ground one of the little posts and run the fuse tap to the other. Absolutely put a fuse or circuit breaker near the battery for safety.