Every time the ignition is turned off continuity is broken on the starter feed wire.
Kill switch relay wiring diagram.
The trickiest option is an ignition kill switch.
Ford to another relay when the alarm is armed and the ignition is turned on.
Once the wire is cut you can either solder the 2 ends of the wire into the switch or use additional connectors and crimp it on no soldering required.
This is the most commonly used application for disabling the starter.
Using your wiring diagram find the wire responsible for activating the starter solenoid and install a switch as far from the ignition as possible.
Normally closed starter kill.
This is a stand alone starter kill.
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Starter kill passive with switch relay wiring diagram.
Below is an ignition kill switch wiring diagram.
It does not rely on an alarm or keyless entry for it to work only a simple momentary contact switch normally open to deactivate it.
By using a wiring diagram for your car you should be able to find which wire is your ignition cable and determine the proper one to cut into.