Same boat, different car, just as unhappy.
Had an iPhone 4S. iPhone music NEVER worked with the Bluetooth in my '09 Mercedes (CLK 350, but I doubt the specific model makes as much difference as the year may...). (Phone? Yes. Music? Never, via Bluetooth.) But the iPhone play list *DID* work perfectly well with the USB jack in my car.
Went to iPhone 5S with the Lightning jack. Put a Lightning adapter on my existing old iPhone 4S-type cable, again, worked perfectly...
...until OS 8.x. Thought maybe it was the old-iPhone-type-cable. Had that replaced. Put the Lightning adapter cable on that, NADA. ZIP. CRICKETS.
OS 8.3 isn't changing anything. (So the most recent posts here that say 8.3 solved their problem? All I can say is, I'm GREEN with envy!) I *KNOW* the car is still communicating with the phone, because throughout this unfortunate debacle, the phone's calling features have worked flawlessly, via Bluetooth, with the car, and the car's dash and steering column controls for my phone work equally flawlessly.
Plug in the USB cable for music to play my iPhone playlist? "No iPod" on my dash, and "no input" on my stereo display.
Tunes are really important to me when I drive. I *hate* radio. (And internet radio is getting just as undesirable as broadcast, IMO.) I am very frustrated, and seriously considering throwing my iPhone out the car window at 80+ mph, then going to buy something that WILL work with my car.
If anyone has an answer, I'm all ears... Have been ever since 8.x destroyed my in-car listening experience!