This seems like someone is not thinking through the whole issue by just comparing the power levels.
This is basically the same as saying: "This hair dryer uses this much power. My furnace uses much more power. So I could save energy by using the hair dryer to heat my house instead of the furnace."
Well, no, it's consuming less power because it's producing far less heat! You would have to run the hair dryer tons more to get the same warmth. The built-in heater of the car, and the 12V heater you're talking about are both using a resistive wire to do a straight resistive conversion of electrical energy to heat energy at 100% efficiency. There is no difference at the heating coil. Granted, the only difference is that the 12V heater is producing the heat right there inside the car in the footwell, so it's not having to transport/blow the heated air anywhere. The car heater is elsewhere and blows the hot air through ducts into the car, so there is a little loss from warming the walls of the ducts, but that would probably be a pretty small effect. But no--there's not going to be an energy difference to get the same amount of warmth.