How do I wire in blue charge wire in truck end.

My dump trailer has a green LED light in it that shows when the hoist battery is being charged from the tow vehicle. Neither my F350 or F550 charge thru the 7 pin flat blade connector. I know the trailer charger works because when I hook up the Chevrolet 1/2 ton the green light glows. The F550 has had the plug added and it does not have the blue hot charge wire at all. Is this simply a extension from the battery/alternator that gets shut off with the truck so it does not continue to pull...

How do I wire in blue charge wire in truck end.

How do I wire in blue charge wire in truck end.