On some German cars the permanent live and ignition live are the wrong way around. If your head unit wiring harness has red and yellow wire you need to swap them round. Most have bullet connectors already so just swap them round. If they don’t have them then cut the red and yellow wire in the middle and fit bullet connectors to swap them that way.
I haven’t changed my head unit but as it’s German I’m convinced that this will be the problem. If that doesn’t work upload a photo and I’ll work it out for you.
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