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Have these been wired with a aftermarket connector for trailer lights?
Have they checked to make sure it is not just the indicators on the dash? That would indicate a cluster problem.
Sorry for my delay also, I would say it is a problem with the trailer wiring.
I cannot say specifically but with both doing the same thing, the odds are just too long that both vehicles would have the same problem without the same cause and that has to be the trailer light installation.