To choose an electric car heater, decide whether you plan to use the heater when the engine is off or running, whether there are combustible materials in your car and how you want to heat the area. Your general options are a 12-volt plug-in heater or a 120-volt space heater.
A 12-volt electric car heater runs while the engine is running, which means it does not rely on power from the battery and does not drain it. Because it provides heat the moment it's turned on, it keeps the driver warm as the factory heater warms up. It can even act as a replacement if the factory heater dies. When used as a replacement for the factory heater, however, a 12-volt plug-in heater relies on more electrical power than a factory heater.
A 12-volt heater may not be safe to use in cars that have combustible materials, such as paper or non-flame-retardant fabric. It's important to choose a 12-volt heater that designed for use in enclosed areas, but fire risks still exist. If your vehicle has combustible materials, it's advisable to use an oil-filled space heater instead.
Convective heaters, such as oil-filled heaters, heat the surrounding air, while radiative heaters only emit heat to the object it's resting on or pointing at. This makes radiative heaters great for use on surfaces that are most affected by the lack of insulation in cars, such as seats and windows, but poor for fire safety.