Is Automobile Power Viable for Electronics Projects?
I recommend using an Adapter from Cigarette lighter to USB (like a cell phone charger), to get your 5V.
It is simple, cheap and effective. And if you tear it open you will find a linear Drop Out Regulator (for the lighter loads) or a switcher (for heavier). And they are typically already current limited and such. So I suspect they are perfect for your goal. I would readily run an Arduino such.