Hi,
A simple voltage divider will do it.
Take a 1.8k resistor, connect one end to your 12V and the other end to a 1k resistor. Take the other end of the 1k resistor to ground. Now connect the PIC's analog in to the juction of the two resistors. Both grounds must be connected together (battery/car and the PIC)

13.6V over the total resistance of 2.8k will give you a current of ~4.86mA. (Ohm's Law, I=U/R or 13.6 / 2800). Those 4.86mA will 'produce' a voltage across the lower resistor of 4.86V. (Again, Ohm's Law, U=I*R or 0.00486 * 1000 = 4.86V). You may need a small cap in parallel with the lower resistor or do some low pass filtering in your code. That's it.

/Henrik Olsson.