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This is an oft asked question....
it's a black plug with a black cap tied with a black zip tie on the black wiring loom running down the dark side of your black radiator support...
on some it runs down the passenger side, on others it runs down the drivers side...
all that I have seen are in the front and zip tied about the level of your tow hooks....
VERY hard to see, but it accepts a three prong grounded plug...
That's where it is on a 7.3L truck, but people were getting too good at finding them, so on the 6.0L trucks Ford found a new place to hide the plug - it's near the passenger side tow hook now. (And I guess too many people were still finding it so now they hide it on the optional equipment list for most states!)
My plug broke on the end so I put a large yellow replacement end on it and moved it above the bumper and below the grill nice and snug. When I get around to it and find a good place to put it, I have a body mount plug. Aftermarket type simular to some larger trucks.