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Welcome, I'm Chris (aka Moose), my normal schedule online is around lunch and after dinner till early morning.
The flashing light is the PCM telling you there is a concern with the transmission and codes are stored which a scan tool can retrieve. This is not related to the wire harness for the trailer.
If you know the codes stored or have any symptoms I can help more as to what is wrong and how to address it. One thing to look at is the trans oil temp reading, that is a common failure part in the 5R110.
Please read this:
Thanks for using Just Answer, Chris