As long as the alternator self-energises any thing, all circuits are not dead. I ground out the ecu key wire, the alternator field wire, and the fuel pump wire, with a immobilizer relay. Imagine that, I use a relay that's already in the wiring, and on the car.

The rule flexibility is because not every car has a coil wire, or it may be a fuel cut-off valve on a carburetor, or in my case it's the ecu wire. Some cars didn't come from the factory with an immobilizer relay, but they're easy to get from your local alarm/sterio shop.