It sounds like you've taken everything out of the equation other than that RCU. If (and that's a big if) you can somehow bypass that too check and see if it goes away, that would be my next step. If you can hook your phone straight up to the aftermarket amp temporarily for a test. Using that as solely to get your source signal from, and that fixes the issue completely, then you know 100% that's the issue.
Anything can get damaged or go out at any given time. That's just life right? Nothing is 100% perfect.
They make RCA jacks to"headphone plugs". If you can get your hands on one of those. Our they sell them all day on
Amazon. It's one more thing to buy, I know, but they're fairly inexpensive.