> Also don't forget that seat heaters are also resistive and most cars in cold climates have them.

I know it (or more like my bum) too damn well, though I didn't think about them in this case.

> small diesels like VW Polo

Makes sense, though as I understood, it's strictly auxiliary till the engine is heated aaaand maybe some additional heating at the parking lot with the engine turned off, though I would wary of this usage, it would drain even the extended battery fast.

Overall it's not quite what I had in my mind (think of space heaters), though yes, there are some parts which can operate independently from the engine, though it's not a primary function for them.

Thanks for the hint.