getting interrupt from ignition circuit
Easiest would be to just share the hall sensor (Pin 6&amp;7).
Pins 5&amp; (7/8) would be the same. The Circuit looks as if there would be 12V vs Chassis Ground on the Hall, so you would need to use a resistor divider on the arduino input (~3:1).
Given the general noisy-ness of Iginition Circuits I would recommend either isolating this (optocoupler with matching resistor) or using TVS Diodes as protection as well.