Looking at this picture of the main logic board, there is wiring for each jack. What is under the cover is uncertain.
picture of logic boardHowever, if I was trying to trouble shoot it, and had one leg that did not work, I might be inclined to take the wiring off of the board for all four jacks, and then use a set of jumper wires to connect to a different logic board output. If the jack works, it's something on the board.
If it doesn't work, try connecting the wiring from a working jack to the place on the logic board where it was not working. If that jack still works in the "bad spot" on the board, the problem might be in the jack itself.