Switch that interrupts the ground circuit

RedFish said

Assuming the fuel pump is powered by a relay... interrupt the relay coil signal. Smaller gauge wire and lesser grade switch req'd.

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THIS RIGHT HERE.

There are several ways to "sneaky" anti-theft some in plain sight

Guys used to advocate wiring the cig. lighter to the tach wire. Push in the lighter--if it's clean--grounds the coil NEG. Does not work very well on MSD LOL

If you have any other relays in the car, for ignition circuit, etc, same deal, interrupt the coil circuit.

You can ALSO buy electric fuel cutoff solenoids (for mechanical pumps)