There is no standard method. Every car is different. You might use the 12V after contact voltage (only present when the contact key is turned on) A rather easy place to find that signal is at your car radio. Most of them have a buildin system to switch the radio off when the ignition is turned off. Not sure how many amps you can get from that wire, but if your power converters are switching, you will only use half the amps at 12V that are needed on 5V. Car technicians usually have a light with a sharp tip they can stick trough the isolation of a wire to see if it carries 12V. Modern car's have electronics that are interconnected using the can bus. They don't like it if you place your test light on one of the can bus data lines. Best method is digging up a cabling plan and using that to find the proper wire.
Messing with the wrong wires can even trigger your airbag if you are extremely unlucky.
I use as a portable power supply an old Perel jumpstarter (12V 4Ah lead-acid battery with 3V, 6V and 9V outlets. I drive an UBEC 5V-3A DC/DC through the 9V outlet, the 5V wires screwed to the PiFace screw terminal (and no micro-USB power supply connected!), and this gives ca. 7 hours of operation. Perel still sends jump-starters (see: http://www.perel.eu/products/list/?id=387488)
I am hoping to power my PI and hotspot from the car fuse box. I have 2 TSR buck converters from adafruit to deliver 5V and 1amp to the devices, but wondered if anyone had a good resource to look at for getting 2 amp from the car fuse box that I can tie into these buck converters. I was hoping there may be a easy to use tap or something, but any advice would be greatly appreciated.
Thanks in advance.