1999 Cadillac Deville 1999 Cadillac Deville electrical prob

By nothing, there is no chime, dome lights, or any indication of battery power at all?

Have you observed the vehicle with this condition, or can the condition be duplicated? There are some checks that can be made, if it can be duplicated it would be best. Initially, in reviewing the schematic, I think that it is loss of power to the IPC that may be causing it. The resulting no start/crank is because it is a key player in the theft system. The cluster fuse in the rear compartment fuse block receives power from the body 3 fuse in the underhood fuse block, via a combination of power relays, controlled power in rear compartment relay center 1, and a controlled power backup relay in rear compartment relay center 2. Both of these relays are controlled by the PZM or body control module. If you can get it to fail, Check for power on the cluster fuse, as well as the radio/phone fuse. If you want to try something and the customer is O.K. Doing the R&D, swap the controlled power and controlled power backup relays both, with similar 'known good' relays, and see if the fault reoccurs.

The rear fuse panel is accessed in the trunk, behind the parcel shelf/rear seat liner. There should be a legend beside it. The relay centers, there are four of them, are behind the rear seat back on the passenger compartment side. They are 1-2-3-4 from left to right, when looking toward the rear, #1 being closest to the center of vehicle. I have not found a breakdown of the relay centers in GM service info, there may be legends next to them, I am not certain.