I know I had one of those, I had one on a shelf at the back of the workshop, only to go & it was not where I expected it to be, nor does it seem to be anywhere else. All I can assume is that on one of those days when stuff exceeded space it was sacrificed to the scrap god. Sorry
Edit:- Not particularly cheap but have you considered
https://www.carbuilder.com/uk/compact-car-heater