It's a special powerssupply called "switchmode". The basic idea is that you generate an AC wave from the car's battery DC, increase it's voltage via a (usually small) transformer and then rectify it again with a diode bridge to get DC again, but at a higher voltage. Usually the AC wave is square and high frequency, so the efficiency of this kind of circuits it's rather high (90% is not uncommon), yet, they're NOT trivial to design or build.
The pdf is from On-Semi (formerly Motorola) of a high current, high voltage "off-line" switcher. Off-line switchers, as the name implies, take the juice right out of the wall socket and convert it to DC. The reason that I mention this one is that a PCB layout is given. Furthermore, you can omit the rectifier on the input since you will be taking 12VDC from the battery, and of course you will have to change the transformer, etc.
Before Starting to build a "not trivial" switchmode PSU,
you might be interested to know,
that there are a bunch of Step up Regulators
that come in form of ICs.
They can be for one voltage or be Adjustable.
LM2577 PDF download
LM2588 PDF download
just paste those two lines, one at the time,
into Google Search, http://www.google.com/
the first is for max 3A max 37 volt out
the other can deliver 5A
I am sure the other guys here can give some more ICs