Simplest way to use Arduino in a 12V system (car)

Hello, I will be powering my arduino off of my cars running 12v system which ranges anywhere from 12V-15V. MY question is, what is the simplest way to power the arduino? Currently its running off a series of AA batteries to 9V. I am thinking an inline 12v to 5v step down regulator/converter will work best. Would a 12V to 9V be better/worse? Is there any other opinions? I have provided a link to the step down converter. I will also be using an inline 3 amp fuse.
Anything helps
Thanks!

http://www.ebay.com/itm/1-MINI-ATC-INLINE-FUSE-HOLDER-20-GAUGE-WIRE-With-3-AMP-FUSES-Automotive-Acc-/321951057813?hash=item4af5c74795:g:tJsAAOSw~gRV2HNY

I think the simplest way is with a 12v car lighter socket usb port. I can get them in the dollar store here for 3 dollars (Canadian). Just plug into your lighter socket plug usb cable into arduino.. done!

You could also feed 12 to a DC to DC SMPS then feed o/p to Arduino input jack or if the DC to DC was +5 feed to standalone circuit.

DC to DC eBay
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