hope i'm not resurrecting too old of a post; i did this on my 2008 4cyl mercury milan today; exactly what the last poster mentioned about that metal panel and box.
the box has 3 nuts holding it to a second bracket, that second bracket is welded to the metal panel covering the access hole to the firewall. Once you take the bracket holding the mitsubishi box off, it has three plugs, you can move it to the right side of the steering column, remove two bigger nuts, and you'll see the magic white panel mentioned above with a bolt attatched to it (the bottom bolt of the second panel you removed. You shouldn't hurt anything removing these.
to put your cable through, i snaked my cable through a hole i drilled slightly smaller then my cable in the white panel, then poked it between the rubber lining inside, to the hole where all the stock wires poke through by the cars computer. You will need to take the battery out to access the white panel under the hood, have someone hold it so you dont drop it; while undoing the tabs inside the car, behind the metal bracket you undid.
i havent found any help on forums till thread
hope the photos help