1991 300E

Hey everyone, im trying to find an adapter to either put a CD changer in my car or a new head unit..

I know my sound system is a higher end option on mercedes of my year, there were to types of higher end systems, i know one of them was called bass reflex and the other im not sure of.
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The round gray plug is what makes everything so difficult so i cant just cut and splice like i normally would. I have attached pictures of the sound system amplifiers and the control box in the passenger side of the trunk behind the carpet shroud...

If you notice there is an open round port on the control box right next to the round gray plug.. Is this a standard plug for a CD changer??

What are your recomendations for either a Changer or an adapter for a head unit.. I know that finding an original Becker Silver Tone Changer will be a very rare thing,,, any suggestions??
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