look at a adjustable voltage regulator...
that with a couple of caps and resistors (maybe $5 worth of parts)
the first diagram in this link should help you out. Just take into account that your car probably puts out more than 12v and should be metered to check. Thats why the pot(R3) is included.
there may be another solution like a resistor, but you would need to meter your batt with the car running and use Ohm's Law to figure out the resistor, but it is inefficient and may cause problems down the road due to heat.
(if your voltage is 14v and you want to drop down to 9v i think it would be about 250 Ohms resistor??? i think the wattage is a 0.1 watt resistors?
Maybe someone can correct me if im wrong but...
V=I*R V=5 due to the difference in voltage
I=.02 amps so....
that is 250 ohms for the resistor and to figure out power dissipation
(I*I)R=P I equals power consumption
again this system would require a 14v system and would change and not be as consistent as a voltage regulator.)
Hope this helps