I bought the Ford "Hands Free Prep Harness" from here:

http://www.parts.com/partlocator/index.cfm?action=searchCatalogOEM&siteid=213787

I got it for $38. It took about two weeks to arrive.

Here is what the back of my 2004 Eddie Bauer Expedition in-dash 6disc changer stereo looks like:



The green wire (the very most top left wire) of the bottom right plug is where I tapped into for the constant power source.