Hi,
Can I suggest you list;
ALL your INPUTS, noting if they are ON/OFF or an analog value.
ALL your OUTPUTS, noting if they need to be ON/OFF or anaog.
Note:
Your DC levels will be as high as 14.4V, depending on battery condition and temperature.
Analog output in most Arduino controllers is a PWM output, not an actual variable voltage level.
The circuit suggested for your ON/OFF inputs is ideal for automotive applications.
Can you tell us what you aim to have as the method of displaying your car information?
Thanks.. Tom... 