I'm not sure what your application is, but if it's an alarm or similar and needs to work when the vehicle is on or off, there is more to consider. You mention an input trigger, but not how you are powering the PIC. Inputs can not go higher than VDD, so this implies your voltage divider needs to be less that VDD when charging (as high as 14.4 V) and when the battery is quite discharged (about 10.8 V). You can do this with a voltage divider but there are a smaller set of values that will work. [R2/(R1+R2) = 2.85]. Finally, there's no need for zeners - a voltage divider will do it all. Zeners are useful where you need a constant voltage regardless of load. That is not your application - the load never changes if it's just the input of the PIC. You can make a zener work, but you are just wasting electricity (and of course money).