I don't think a ballast resistor is the way to go, the voltage drop from the ballast resistor (Rb) is proportional to the system resistance (Rs). According to ohms law, I=E/R (Current equals voltage divided by resistance). If the systems resistance were fixed it would work, resulting in Ib=Eb/Rb and Is=Es/Rs (note that Ib=Is if Rb and Rs are in series). But in reality the system resistance varies with the load (switching on/off of components; lights, wipers, radio, etc); also the voltage varies (unless well regulated), and the current varies.
If you use a 12 volt battery and charging system, you could use a properly sized (current rated at ??) voltage regulator to operate the 6 volt components.
This would require some modifications to the wiring to segregate 12 volt circuits from 6 volt circuits, but properly done it would be safe and cost effective.