How do you run a 5v MCU circuit on a 12v car battery?
Current is always drawn, not supplied: that is, connecting something that can supply a lot of current does not mean that it will.

The standard 5V linear regulator will work fine, but it will dissipate power as heat, proportional to the amount of current drawn by the PIC. I'd suggest something based around a "switchmode" power supply, or cannibalising a car USB power converter (cheap and widely available).