Make sure that what you're proposing is safe and legal before you start. At a minimum, you would need to have a neutral lock-out so that it could not be started in gear. Here in the UK it would be illegal since it is not legal to leave a running vehicle unattended, and if it had been legal I would have expected insurers to be extremely interested in a modification which makes it possible to start the car without a driver in it, and enables somebody without a key to start it.
Once you have overcome those issues, in order to get it to work you will also need to disarm whatever alarm/immobiliser your car may have installed. Not a problem if you don't have one, but most cars do these days.
How could I tell?
And if you just want the car to be defrosted and cosy on a cold winter morning then a plain old domestic fan heater on the back seat powered by an extension lead works fine for me. The installation cost is zero and it works on whatever car I happen to be driving.
And even in the UK, I suspect electric heating is cheaper than petrol.
A close call, I suspect.
also, why would you consider leaving a car to "warm up" while running idle a bad idea?
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