The Arduino board processors have WDT and Brownout protection but they have to be enabled and perhaps setup.
Automotive power is not very friendly, you will have to protect from:
- very short 1000+ volt spikes.
- 100 ms of 50V (look up load dump)
- Reverse voltage (meant if someone installs the battery backwards or maybe jump started it backwards)
- if a gasoline vehicle, the startup voltage can cycle between 5V and 8V on a cold day when the engine is cranking.