Is this the same switch matrix i had on another thread (http://aussiepinball.com/index.php?topic=341.15) a couple of weeks ago - IC11 the PIA on the driver board?

When i tested the IC17 and IC18, they were the opposite to what you suggested.

Is there a way to test the switch matrix itself or is that done through testing the other chips i have already done?

But it is all very strange that if this is the problem, you would think it would have had the non illumination probelms all along rather than just popping up today...although stranger things have happened!