with scan tool hooked up you need to start unplugging every module on the bus one at a time and hope like L the communication comes up. That will often give you a general direction in which to go.... BUT!! On many occasions things might come alive, you reassemble, and all continues to work. Seen this more times that I care to think about. I had a truck of this era experience this and I unplugged the ABS module and the cluster came alive and communication was restored. We drove the truck a couple of days with the abs module unhooked and the prob didn't reoccur. Plugged the module back in and it still didn't act up. It was a regular customer and it went for years and didn't ever happen again. Things I have seen.... crank sensors, the solenoids in the 46re trans, seat belt module on 99 only pickup.... could be a lot of things.