Re: 12 volt down to 6 volt.
For maximum efficiency, use a variable switching regulator like the LM2575 (adjustable version). It needs very few external components- the schematic is provided in the datasheet. If you want something "quick and dirty", go with an adjustable linear like the LM317. It will heat up more (wasting energy), though. I say adjustable because these regulators don't come in a 6V version, although I'm sure you can find one that does.
Another benefit is that you can vary the brightness of the lamp by adjusting the voltage.