some makes of car already have an auxiliary heater, fuelled by diesel from the tank, that
"operates at low ambient temperatures to boost the engine coolant temperature to improve engine starting and cabin heating"
I have never wanted a diesel car so have no experience of them.
I understand Volkwagen Beetles used to have something similar available as an accessory in the spare boot at the front.
Perhaps future electric cars will have a little charcoal stove, or a paraffin heater, for the winter. Or maybe a chemical heat storage block that you heat up when you plug it in to charge the battery would be simpler and safer.