Royal Oak, MI
I can't get to the link here at work, but marvin's pinball guides will walk you through how to test if its on the board or in the wiring. Bascially, you can just build a very simple circuit with alligator clips, a bulb and a diode and connect that to the board and test each row/column.
In my most recent case, I pin pointed it to the board with the test above (light would not work when connecting to that row). Then I tested the board's resistors and diodes that are downstream of that pin with a meter and pinpointed it to a single diode.
Hope that helps!
