I want to keep my 6V system and buy your 12V fan and heater core

We are not electricians.  We collect clever work around solutions from automotive enthusiasts to help automotive enthusiasts.

A customer told us his recipe for using a 12V fan in his 6V system.  

A ballast resistor allows an ignition system to operate at a lower voltage until the engine starts. The ballast resistor then functions to regulate the voltage (4-6V)  going to the ignition system to avoid additional wear on the system e.g. burning up points.  

That's the recipe.  All of it. 

More on ballast resistors...

Ballast resistors adjust their resistance based on the current. As the current exceeds a threshold value, the resistance increases, and it decreases when the current falls.

In this way, the ballast resistor tries to maintain a constant current flowing through a circuit.

Unlike a load resistor, which has a constant resistance under varying currents and voltage, a ballast resistor acts as a variable load within the system, adjusting its resistance as needed.