Ive just repaired my M3 with trailer connector. It was full of water from rains. Causing the trailer symbol to appear, rapid indicator flash on the display. I was able to repair it from underneath, completely stripping down the connector, drying it out and where the wire plug into the front part of the connector applying gollops of di electric grease to seal out water ingress. I also drilled small holes around the edge where water could easily sit.
Problem has gone away- hopefully for good