88 Toyota Pick up Inst. Panel lighting problem

I have an 88 Toyota 4x4 pick up with an instrument lighting problem. The dash lights just quit working 1 evening. I checked the fuse, it looked good but I replaced it anyway. The tail lights, parking lights and liscense plate lights all work. I checked the on-dash rheostat switch, it appears to be working fine and I already replaced the tail light relay. Can't seem to isolate the problem. My Toyota book wiring diagram doesn"t even show the rheostat wiring. When I tried jumping a Hot lead to the white/black wire off the rheostat it sparked - like a short. Problem is, this wire disappears into the harness - can't trace it. Any help out there would be great. Thanks, J. Bryan