How do you drain water from the fuel system on a 2004 Ford 6 liter diesel?

Inside the left frame rail, under the driver's side, is your water/fuel separator. It's where your primary fuel filter is. You'll need a 6mm wrench to open and drain the water. Look on the lower part of the separator for a round knob or disc with a hex in the center. If you have a two wheel drive, it'll be a simple task of sticking you head up and looking for the hex hole. If you have a 4x4, you'll have to bump your head on the front drive-shaft to find the hex hole. Also, you'll fine the type of Allen Wrench with the ball end is most helpful to loosen the drain knob if you have a 4x4.