Stainy

I'm fitting a simple anti-hijack system to my MY04 WRX. The wiring looks straight forward for the horn/indicators/ignition etc. but I need to know which is the best circuit to use for the immobiliser.

The system 'pulses' the engine before shutting down, so I presume a fuel-cut is out of the question as the pulses would be too short to notice.

Any ideas on which circuit I should be looking at cutting and where it's located?

(It's only for anti-hijack purposes so it doesn't have to be hard to locate etc, just the opposite in fact)

Please PM me if you don't want to answer this in open forum