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Yes that explains it. When cold, the rear lights flash on every other front light flash. In this case I would suspect the single point fault source as the BCM itself. I think it is the driver in the BCM rather than the rear light assemblies. The only way to verify is to do actual troubleshooting with a multimeter, or stick with the usual dealer goon method of just throwing parts at it. That way the goon at least gets paid to replace parts.