The supermarket car wash is liable for anything that happens to that vehicle whilst in their control (not just the car wash). I would be checking to make sure they hold insurance prior to passing my keys to anyone. Whenever you pass the control of your vehicle to someone else it is a risk. Personally, I would never use one. But, if I did, I would want a written contract in place before I did. This should either be by the car wash having a visible sign with a copy of their insurer visible to the public or a contract that you sign before you hand over control of your goods to them.