Hi RossGard,
The problem you are experiencing seems to be with the Tuner pack in the rear quarter panel. If you had sound but lost it all of the sudden I would say you may have lost a fuse. But if the sound is just not there and the fuses are good then I would suspect the tuner pack 1st. The amps are seperated, one for each side of the car, front and rear. Normally you would not lose both at the same time. The tuner pack on the otherhand controls all audio being processed and therefore is more likely to be the problem. If you were losing communications with the Head unit you would get an error code of "E 2". Let me know if I can help you in any way. We service these units and can get the problem confirmed and repaired for you.

Cleeve Morrison
La Jolla Audio Repair

www.mercedesradiorepair.net