input to pic with 12V and resistors
All you need is one resistor about 100k in sereis with the signal.  the PIC has internal diodes that will protect it from the current that flows in the resistor. the input will appear high for voltages above about 2 to 3 volts, if you want to increase that threshold to around 6V then adding a 47K resisto from the GPIO to ground would do that.

LM7805 can swallow a few milliamperes of back-feed so if the pic isn't using the excess signal current the 7805 will deal with it for you, if you're worried put a 5.1V zener parallel with the PIC.

However I don't think 7805 is really suited to automotive applications, the voltages can be well above 40V