Sensing Output From Spark Plug Cable Of Petrol Engine

How can I sense induced voltage by wrapping a wire around the spark plug cable of a petrol engine. I have a a car alarm system that can remotely start my car. It came with a device that has a wire that you wrap around the spark plug cable to tell the electronics that the car has started. This system is sealed up by the manufacturer so I can't access the cct. I need an output that I can amplify to drive a 12V relay. Anyone with an idea? Thanks

A couple of turns of 1 mm² wire around one of the sparkplug leads will work fine.
Leave one end open and the other end goes to your BC547.
Bring that to the base of a BC 547. fit a base resistor 47k. and a protection diode + resistor to negative to protect your base from overvoltage.
A bit of trial and error may be needed, see what works best.
Than amplify signal to whatever circuit you want to drive.

I used it to drive a LED and a pulsecounter in my old Ford Falcon.

Thanks, I'll try that now and keep you posted