In high school, one of my friends had a VW (air cooled) with a torpedo heater installed that would heat the interior to 30 degrees C in a matter of minutes - no matter how cold it was outside.

Likewise, in university, my friend's army jeep had a gas heater that would melt snow off his canvas roof.

People are consuming 30% of their battery power to heat the vehicle and batteries - and that's with reduced battery range. It only makes sense to add a container for gas/diesel and a heater - really can't understand why manufacturers are against instant and very low cost heat as an option to protect range and increase safety (in case you're stranded in the winter with a depleted battery).

Best of luck adapting one to your vehicle.