Removing and re-adding same sprite to list Unity
Solution 1:
You have a small flaw in your logic here. Saying that you add the car sprites in the inspector, removing all the sprites is not ideal.
You can have a list of all the sprites, and a list with available options (starts with sprite[0] and adds each time you complete the level).
However, if you want to know how to add them back after deleting them, you need to make a copy in a global variable, so it's more efficient to try what I am suggesting above.