You would have to have something around 2000 Watts of heat to do a decent job of heating the cab of a vehicle. You are not going to be able to pull that kind of power from your vehicles 12 volt system.
2000 Watts / 12 Volts = 166.66 Amps
So along with all the current you alternator has to supply to: recharge the battery after starting, power the engine ignition, (and fuel pump if it is electric, and fan if it is electric, and fuel metering system if it is electric, and engine computer if you vehicle has one (1996 or newer probably has one), and any other electric items such as lights, sound system, seats, windows, navigation) very few vehicles have an alternator with 166 Amps of unused capacity. Most have very little reserve after all the items of the vehicle are supplied power.