A friend has one fitted to his R###e R###r (
sorry for swearing) and tells me it is incredibly efficient, and very cheap to run.
He has a separate battery system for it, and can remotely fire it up to heat/defrost the car without starting it.
It is so efficient he is going to rig one up in his barn of a garage, with ducting to direct the heating strategically, and possibly use red diesel to reduce costs.....