/diy/ - Do It Yourself

I posted this question in /o/ and nobody could help, and I tried /g/ but nobody bothered with a reply.

I'm looking to add an Arduino based keypad/codelock ignition system to my car, but I also want to keep the old key system in place as a backup incase the Arduino fails for some reason.

I've looked at the circuit diagram of the ignition system and it looks like an on/off switch, but I know it's not that simple, because the key system is supposed to use 2 switches, one for the electronics, such as the fuel pump and sensors and such, and the other for the starter motor.

I don't know much about cars, I can service it and change a tyre, that's about it. I'm a programmer by profession, used to be a welder (not a very good one at that).

Does anyone know if I can add an ignition system like that? If so, where should I be looking and what should I be looking out for?