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I wired a kill switch into my car using the crank sensors wire that runs into the ECU.
It does throw a *check engine light* on the VW but it will not on all cars (check on a Fiat Bravo forum for the preferred method)

2 wires run from the sensor under the hood to the ecu ,your wiring diagram will give you the color code to find the wires in the loom , expose the loom sheathing just ahead of the ECU itself to search for and splice into and solder in some wire and a on/off switch, mounted somewhere in a inconspicuous yet convenient place to use.
But
You could also wire in a fuel injector kill switch , which would shut the motor down as well.
Or wire into the ignition , which may work the best way to avoid a check engine light.

With the crank sensor kill switch I did to my car it causes a check engine light Plus I need to cycle the ignition one turn back then forward( a issue caused by VW's security ignition system)

If I was to do it again I would use a ignition kills switch in hopes the *VW* would not need its key cycled to start again on the next jump start.

I suggest you check the Fiat Bravo forums for the best method.