This was addressed earlier but it bears repeating.

Those nice 110v ceramic heaters crank out 1500watts of heat, 
equivalent to, i think, 5200 BTU.  To achieve the same heat output at 
12v would require, in terms of amperage,  1500watt/12v= 125amps.  
That's ALOT of juice folks.  Electric heaters from a 12v system are 
not practical in an automobile context.  

sorry to burst the proverbial bubble...