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That can be a bad connection at the speakers OR at the source OR a bad line that may have been damaged during construction. IF this has worked at all then chances are good it is just a bad connection. Only some testing with a multimeter can tell you for sure IF all the visual inspections prove negative. Hope that gives you some places to check. Please get back to me if you need more input from me as all follow ups are included in our fee.
Just checking back in to see if you were able to resolve this issue or do you need additional input from me here?