With a factory wiring diagram it’s pretty easy to trace what the ignition switch does in the PDC. All you have to do is simulate that. I did it with a toggle switch to the ignition relay, and a start button on the starter relay. The entire “truck” side of my harness was clipped at the firewall connection. I had to add back the diagnostic plug but everything else was clipped. The whole “engine” side of the harness (with the ECU) is basically stand alone and operates perfectly independent of the rest of it. The only 2 codes I had were for a battery temp sensor, and evap.
