This is a 2003 Cadillac DeVille-DHS. The customer said that the tach and speedo had stopped working while driving. After some diagnostics and research, I found that the ignition 1 fuse had blown. Time to find the shorting circuit. It was necessary to isolate 7 circuit feeds from the ignition 1 fuse. The fault was intermittent and it took some time to isolate the 7 feeds down to the shorting circuit. Circuit isolation is the only way to pinpoint this type of fault when a single fuse feeds power to multiple components within the system. After narrowing it down to the rear integration module (RIM) and the rear object sensor control module (ROSCM) circuits, I was able to find that the harness rubbed through in the right rear sail panel pinch weld area and was shorting the power feed wire to the rear object sensor control module. The scan tool had communication with the module until the fuse would blow. 4 modules would stop communicating. The MIL lamp would also shut off. This looked like a PCM fault at first glance and very well could have lead to false PCM replacement, if not for diagnostic due diligence.
This one was a true automotive bug hunt.