For some reason, whoever installed this unit for me probably didn't do it right. I am not sure if there is something wrong with my headunit also.

When I playback music from FM radio, SD, DVD Movies. I can only hear the sound from the front, driver side and passenger side speakers, the rear has nothing.

On the headunit, I check the settings for Sound. The Sub.W is greyout, I can't change anything.

When click on Front/Rear speakers Balance. I can only click left to right. Thats it.

But if I play .avi movies from the USB. I can change the balance to left, right and front and rear. But still, I can barely hear the sound from the rear speakers from both driver and passenger side, almost nothing.

The front passenger and driver side are very loud like normal.

My car is a 2007 Mercedes and used to have Logic 7 surround sound speakers, sub W. is built in at the trunk area.

There are lots of speakers in the car but unfortunately only the front is working.

I have a BMW with an 8 speaker system and as an installer I could say have them re-check the connections. I know you have a Logic factory amplifier in your vehicle and although I normally don't work on Mercedes because of their fiber optic antennas (which my company can't incorporate anything with) cause issues, however my point being is something the RCA's are used to send the audio signal to the factory amplifier, which when split no longer gives you fade. Yet, I really doubt that may be the case here just go back and have the installer double check everything out.