While I was running wires for my XM I came across three unused plugs and I was wondering what they were for. One is right with the radio harness. Another is under the dash near the steering column. And what is that plug that is mounted under the dash right under the steering column?? Funny what you come across when you start searching around.

 

The plug under the steering wheel is where the dealer will plug-in the diagnostic computer, or you would plug your tuner in there.
The wiring harness behind the stereo may be for the Audiophile system, or DVD system, if you do not have those options installed. The harness under the steering wheel may be for the steering wheel radio controls if you do not have that option.

 

Ford seems to use a generic harness when building the trucks. It's quite common to find several connectors tucked out of the way. The little black one by the stereo harness is for the Audiophile sub. The OBD-II connector for diagnostics under the steering column. Etc...