Ok, if you visit the great WHITE north.. In the winter, you will find many 12 volt "Defroster" type heaters, designed to aid in defrosting the windshield they run about 100 watts.. Plug in to the accessory socket
I Do not really recommend them.. but that's about the best you can do.
IF you have an inverter there is "The World's Smallest Space Heater" 150 watts at 120 volt, and they ahve been advertissing "The Wall Hugger" I think it's 250 watts, pluggs into a standard wall outlet but no cord, plug is attached to the housing.