Yes. Cars ARE in fact insulated very well. You only need to take a ride in e.g. my old series III landrover woith nowt but a sheet of aluminium between you and the elements to see how MUCH insulation a modern car has by comparison.
Most cars have something like 1/4" of fibre and carpet on teh floors, the rear seats line that part of the car, and the doors usually have a
2-4" aiorgap in them. Dreaughts are of necssity totally absent by and large, and roofs are normally lined with headlining and again about 1/4" of insulation. some cars even have double glazed windows.Even with a U value of about 5 - equivalent to a totally single glazed car, and a square meterage area of - what - 16 sq meters of cabin, that is 80W per degree C differential, so AT WORST for -5 outside and 25C inside, it only needs 2.4Kw to heat it. In practice that is a fairly ludicrous U vcale, becaseu teh glass is thicker than window glass, and most of the cabin is well insulated.
Lets face it, a lining of 15mm celotex is not going to cost very much. And there is going to be a wedge of batteries and electronics under the floor kicking out a hundred watts. Getting heat from the motors - if thse are ouboard on the wheels - is not so easy, but even there, if wound with pipe istead of solid wire, the heat could be removed bu circulatng coolant through them..