"CAUTION battery voltage too low volume limited"
 

I have no sound coming out of any of my speakers in my car and the cd changer does not work I have a 2003 mercedes sl500 how can i fix this? I don't think there is really any actually problem with the battery cuz the car runs fine lights etc but for the past few days the sound stopped working. The radio and everything else appears to work fine on the screen just no sound from speakers.

I heard the secondary battery in the trunk runs all these items do you think if i charge the battery myself with a wall powered car charger i can restore things atleast for a while?