One of my favorite youtube electronics geeks tears this thing down:

https://youtu.be/R5IKRB_NbGg

here's the item on banggood.com:
https://www.banggood.com/DC-12V-8000W-Knob-Switch-Single-Hole-Diesel-Car-Heater-p-1349613.html

Surprisingly simple once he tears it apart.  If you had a diesel van this
might be a good heater choice.  It doesn't say the power consumption and
that could be considerable with the glow plug.  I made inquiry on that.  It
will be interesting to see Julian's later video when he gets it set up and
running.  I also always wonder about the safety of these products from the
explosively developing technology centers in China.  I've had remarkably
good luck with those products but tend to avoid those that could kill me.
I would definitely have a lot of safety in mind *if* I installed one of
these in a place I would be spending time.

Edward