OK, so you will need about 50 amps or so to be sure you can power everything else on the car, so 150 amps x 12 gives you 1.8kW, which might be OK.
.... but, you'd better make sure you have some way of upping the wiring, as you will need some proper battery cable style copper wire to connect that mother direct to the alternator, or you're gonna get a single use flame-based heater ! ...... sounds like a bonfire waiting to happen.
i looked at an electric solution for a long time, and drew a blank. If you can get a 24V battery system there are elec heaters for crane/truck/coach cabins. Or you can use one of these totally useless 12V fan heaters found in camper vans and atach to the defrost vents.
I first used an old triumph spitfire heater (from ebay), which is very slim, and then changed my mind, and modded the whole arrangement to fit one of these:
http://www.t7design.co.uk/index.php?...64ca520d8e2890