I saw this post and am thinking the original poster has long since moved on to solve his problem. I have a 2007 Navigator that probably has the same radiator. I would like to know where my drain is also. So I looked in my factory service manual and it does not show a drain plug (sometimes little things like that are not shown in the drawings). Then I looked under the hood and also under the truck. I saw a red cap on the passenger side radiator tank. I've never seen a drain capped like this so I googled the question for a radiator drain location.
Here is what I found:
Look in the engine compartment. There is a red **** (cap) near the lowerright tank face of the radiator behind a wiring loom (passenger side). Turn left to open the amount you wish to drain the radiator. Tighten by turning right to stop flow of coolant. Space is tight to get to the red ****.