Are you sure?
The volumetric sensor comes off the 10AS unit (sensor input and power supply), not the ECM. The volumetric sensor will trigger the alarm which will then trigger the immobiliser was my understanding. Same as door and bonnet switches.
From the wiring diagrams the harness may still be there. 3 wires terminating behind headlining above RH B-Piller, White/Black, Brown, Black. Haven't checked mine.

