Hmm, this is not a question of Voltage [V], but of electrical power [in Watt W]. Power is voltage multiplied by amperage, W = V * A.
38 A and 12 V, that means the alternator has a power output of 450 W. The lights may need 50 + 50 + 5 + 5 + 10 + 5 W ~ 130 W, the ignition / electronic/etc. ??? W (100 ? W), the brake light 20 W, so I think a heating equipment with about 100 W would be fine. The normal 12 V electric outlets have a security fuse with 10 A (that means you could use 120 W).
But never think of a air heating system in the car itself, that's ridiculos, and also to transform the voltage from 12 V to 24 V or 240 V, you will just loose the efficiency of the system.
It is better to bring the needed heat just to the right parts of the body instead of heating the air in the open car to heat the body in this car.
Waldemar
please forgive my poor english
