Car bluetooth Problem my year 2009 Mercedes C class can pair with my iphone 5 no problem at all
the car speaker can receive phone calls and I can talk hands free, also when I am using my iphone in the google map nav mode, the speaker speaks well through bluetooth connnection

the problem is when I listening music or watch videos, the speak doesn't work....
A common issue is the output selection in the media apps: 

Make sure that on your phone, in the media screen, you have selected the correct output source (Bluetooth - Mercedes name) to play the audio (assuming audio is coming from the phone when it doesn't work)

If no audio comes (make sure volume is up) .. However, the Mercedes model / year may not support music playback. 

As @tyson mentioned, inquire with the car manual for Bluetooth settings