I had been wanting to add mp3 playing ability to the stereo in my Ford Sport Trac. Happily, I found a device on eBay, and got it for a good price. It is the FRD03-POD/S. Note that the site I bought mine from is now defunct, but this appears to be the same module: http://www.autosoundcentral.com/PIE_FRD03_POD_S_Ford_iPod_Adapter_p/frd03pods.htm
This was a very easy installation! This is how you remove a stereo on many Ford vehicles:
The difference in the Sport Trac is that there is a piece of trim that covers the edges of the stereo, so you have to remove it first. The hardest part of this job was going and acquiring a set of metric sockets, since mine were standard. In my Sport Trac, the bolts were 7mm (8mm for the negative battery terminal). Just the very first part of this video shows a similar piece of trim with the bolts in the same place being removed in a Mercury Mariner. The picture at the top of this page shows the bracket the stereo sits on top of normally.
Above, you can see the black box, which is the control box for this ipod dock, that I mounted to hang down below the bracket. Two of these ports on the back of the stereo, you plug a double-sided plug that comes from the controller box into, and then plug the regular vehicle cable into the double-sided plug, piggyback style
The end of the ipod dock cable where it connects to the controller box:
The iPod dock cable comes into the end of the glove box, where I can plug in the iPod:
You can use the iPod by pushing the SAT button on the stereo. This adapter basically replaces a satellite radio module with an iPod. I never installed the satellite radio module, so this works well for me. You use the seek buttons to cycle through the various playback modes on the iPod, such as artist, album, genre, song, playlist, etc. Then you use the rewind/FF buttons to skip back and forth through tracks. I'm still learning the other controls. It shows the name of what's playing on the built in stereo's screen, and charges the iPod when the truck is running, and does not charge it when the truck is not running. So, if you cut off the ignition and leave the truck, the iPod stops getting fed a charge, and goes to sleep. When you start the engine again, everything wakes up and starts playing on the same song it stopped on.