How can I sense a signal on a 12v automotive wire non invasively?
NVE corp makes GMR (Giant Magnetoresistance) based sensor ICs that can non-invasively sense the current in a wire that is in close proximity to the chip.  Usually they sense a PCB trace running under the chip, but you could perhaps make a PCB with a slot under the IC and clamp the wire into the slot.

They even have a video app note for using their AAL024 sensor with the Arduino.