A zener diode. Your local
Radio Shack has 12V, 1W devices in stock. Put the diode across the clock with the diode's band to the + terminal. Then connect your 12V to the clock + through a resistor. The resistor value should be just high enough to supply current ffor the clock and the diode will clamp the max. voltage to 12V. The zener shouldn't draw any current with the ignition off since the battery voltage will drop back to about 12V.