Keep yourself comfortable when car camping. Make sure that you have adequate layers to keep you warm when the temperature plunges. Bring an emergency blanket or two as well. Hopefully, you’ve brought a sleeping bag that is fit for the season as well.
If you need to heat up water for hot showers, then consider using the heat of your car engine. You can find equipment that will let you absorb the heat generated by the vehicle and make use of it instead of simply letting it dissipate.
Another way to heat up water in car is to plug in a 12V electric kettle in the cigarette lighter. This should be great for your morning coffee, soup, oatmeal, or other forms of quick-cooking breakfast.
You may want to test out your equipment at home so that you can fix issues before heading out.