IMHO Any time something is employed to disable the vehicle, it has a parts per million chance to malfunction and leave you in a dangerous position. So I treat an auxiliary modification with the care and respect it deserves.
Whatever circuit you choose I would check to ensure that by breaking the circuit it could not cause a fault code to be thrown by the ECU.
A talented thief is all-too-aware of the ten top hiding spots for a kill switch. He can run through all 10 in less time than it takes to read this.
A keyed switch with terminals encapsulated in a nightmare grade compound like Liquid Nails can be hard wired in an inaccessible place like under the front seat. Use huge battery cable heat-shrink tubing over the terminals and then for an additional inch or so onto the wires.
Under vehicle wiring is vulnerable. I would use cross-link polyethylene insulated wire, sheathed in fuel line hose.
When this all seems excessive, imagine looking for a long-gone vehicle then out the side window at a triangle of lights approaching on a railroad crossing...it'll keep you honest :)
