Bought one of these heater units for the demisters. It's pretty compact and chucks out a fair bit of heat. Probably enough to keep the cabin warm on cold days. Was pretty cheap too. Perfect if you need to have working demisters like here in Aus. I got the biggest unit, which is 800W which arrived today in super fast time. So I decided to make a quick test to see how well it works.

I connected it up to my benchtop power supply, which can supply 40 amps continuously and has a handy voltage and amperage readout. On initial power-up the unit drew 43 amps, but as the heating element warned up this dropped to a steady 20 amps when running. Obviously it's gonna drain your battery pretty quickly if you use it with the engine off, and you would definitely want to make sure that you have a high powered generator or alternator fitted too, but i was pleasantly surprised at the heat it kicked out.

The unit is pretty small too so is easy to mount. It comes in a pressed steel box so is not modern and plastic looking, which is a definite bonus.

 I'm planning on trying to make some kind of diverter under the dash so that I can swap between heating the cabin and demisting the screen.