OK, I've worked in the automotive world, and there are a LOT of things you aren't considering.
Is this being powered from the car? What type of DC-DC controller do you have on the input of this device to power the uC and other circuitry? Have you considered load dump/BCI tolerance for this circuit? If you end up with a 42V transient at the power input, will it survive? Read about BCI... Implement redundant failsafes so that your vehicle does NOT try to start if a BCI or other transient event makes your microcontroller suddenly think that it's time to start the car. You may want to enclose this entire thing in a shielded box. Implement input filtering on the power input. Put a series diode on the input and a big cap on the PCB side so you can hopefully ride through cold crank and not have your uC lose it's mind when the vehicle starts cranking in a worst-case situation and input voltage drops below 5V. Etc, etc, etc...
Also... Look at the schematic for your vehicle. Where do the wires go that you are using to control the vehicle. How much current flows through them. How high does the voltage ring during cold crank on the pin you're using to sense alternator voltage. Is it high enough to damage the ADC you've connected it to? Are you using a buffer? What is the Abs(max) voltage of any pins that interface to the outside world vs. what the voltage could possibly be.
shudders to think what could happen