Array index out of range after build

Are you sure cars.Length is greater than 0 in the build? How do you populate the array? Can you:

1. Add a Debug.LogError(cars.Length); before calling Random.Range

2. Open your build settings

3. Check the Development build option

4. Build your game

5. See what the console outputs

Can it happen, that the array is not existing at all? I run into same problems like, you create the variable like this: 'GameObject[] cars', and then the program goes a way where it gets no value at all. since its an array it will let you use the [] call, but throws an exception even for the 0 value.