W204 model 2008 with used installed OEM command , all work but no sound

  mercedes/c-class/2008-2015

i had a problem in my original DVD unit ( Navi command unit ) and to fix it at Mercedes Agent they asked for €900, so I bought used head unit ( without the 7” screen ) because my original 7” screen works perfectly ,
after installing the new DVD /GPS head unit everything worked even the GPS , but there is no sound ,
also I can control everything from the console media Knob because non of the new command unit buttons are working!!
now , the electrician who installed it for me said it could be because the new head unit is damaged , while I suspect that I need to program my car Vin number on the new command center!
can you please advice me ABOUT what should I do ?
​​​​​​​have a good day everyone .

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