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I at present have the covering panels of the rear tail gate door and before I replace them I am trying to find out if this stray nickle plated plug has any possible connection to something else . I have added a couple of photos showing the plug and its location amongst the rest of the wiring loom as indicated by the red arrows
When I bought my used 2005 C230, it had poor radio reception. During the process of repairing the rear window shade, I discovered that one of the wires in the radio antenna amplifier had become disconnected. (I also found broken glass everywhere which suggests it may have become disconnected when a previous owner had the rear window replaced.)
Although I have a sedan and you have wagon or an estate (if you are in the U.K.) it looks like you are having the same issue I had. I suggest that you check to see if it will fit the dangling wire into a vacant slot in the device immediately above it with similar wires. If you have poor radio reception now and connecting the wire improves the reception, you will know that your repair worked.