Bluetooth Car Mode Starter is a simple application that allows you to start car mode when your device is connected to a Bluetooth device. Similarly, car mode is disabled when that device disconnects.
Why is this different from other automation applications?
Bluetooth Car Mode Starter ensures that no duplicate toggles are made, this means that if you have an application that detects car mode and does stuff upon that, it will only detect car mode once.
Plus, it's open source.
This application doesn't do anything on Android 4.2, am I missing something?
No, Google removed a huge part of Car Mode in Android 4.2, luckily, workarounds exist for rooted users:
Install Xposed Framework and this http://forum.xda-developers.com/showthread.php?t=2361166