My stock radio in my 05 is the 6 disc/MP3, non-audiophile as far as I know. There are 2 plugs on the back. The harness adapter kit I bought only has the larger plug and seems to contain all the power and speaker wires to mate to the aftermarket radio. So my question is what is the second plug for?

 

The 16 pin port??? I think that's for the rear seat entertainment.

 

Not sure if its 16 but that sounds close. I don't have rear seat controls but I suppose ford might put the harness there just in case.