I used to pull a 40 foot 5th wheel camper trailer with a converted semi tractor all over the U.S. and I wired a big 12VDC to 110VAC converter to 2 or 3 of the 4 batteries in the semi and plugged in a microwave, a TV, an electric blanket and a household electric heater to heat the sleeper when stopped for the night away from shore power. My wife insisted on it!!! So this would be possible to use in a car to make it warm, but you'd be prudent to use a dedicated battery rather than the one used to start the car! The battery in a battery box and the converter would fit in the trunk or on the back seat floor and use a big Deep Cycle battery. This surely wouldn't be for everyone, but for us in the cold winters, it'd be cheaper than a remote starter. I have also had small quartz heaters that plug into the cigarette lighter socket, but again, they can't be left on forever because of the size of the car's battery. Of course this won't warm the engine, but your butt would be warm! I guess it's all about priorities!!! And you'd need a way to recharge the battery, so this leaves most apartment dwellers out and while you're at work!