The only possible work around is a trip to the Mercedes dealer's service department to see if they have an update to the audio system.
Bluetooth is a method of getting data from one place to another - it doesn't specify the format of the data, or what the data is - just how to get it there. If there was an update to the phone, and not to the car, it stops working. (Some day Bluetooth will also specify the minimum standards for the data itself. We used to not be able to send pictures via text between carriers for the same reason - lack of standards.)