If you put a switch in series with the clutch switch or the starter solenoid, this will prevent the engine from cranking. This might be a tip off that there is a hidden switch somewhere. Several years ago a Cummins employee, Josh Berman, answered a question on how to disable the ISB engine from the Cummins Customer Assistance Web Page. The trick was to put a switch in to break a connection to the ECM. Here is a copy of the post:
"In order to insert a 'kill' switch, you will need to put a switch in the 'key switch signal' running into the engine mounted ECM. If you open the switch when you park the truck, and then try to start the truck without closing the switch, the truck will crank but not start, since the engine-mounted ECM will think you have not turned the key on. "
On my 98 Ram the wire I broke to add the switch is in the PDC. It is a green/black wire on pin 28 of connector C130. I also use this added anti-theft switch to get oil pressure up before the engine fires if it has been parked for some time.