Well if you guys have checked everything around the light switch the transistor, the little box on the side of the column support, and power and all that, it could be something in the line that powers the lights up that is shorted, i.e. start unplugging all of the components that are attached to the light dim output of the transistor module. Looking in my FSM I would start by getting the lower dash off, this will cause you to unplug the ashtray light which is one attached to this, see if it lights up, if not carry on to the next which would be the HVAC controls, check again still doesn't work, unplug the radio, check again, still doesn't work, unplug the english/metric switch and the light off button if you have one, check again, still doesn't work then take the cluster out and plug in all the other stuff that you unplugged previously, do they work now, then it is the cluster. That is all that I can add, big pain in the ass to do, but I think that is all that you are left with.
Oh one other thing that is attached to this circuit is the audible warning box, located in the convience center under the dash on the firewall, should be a green box, unplug that as well. Hope this helps.