Amp loudness increases with left/right balance adjustment

[Pound]

Using a LOC to add a single channel sub amp to a factory HU, what would cause the amp to be near silent when balance is centered, then gain to proper loudness when you move the balance to the left OR the right?

[MLA]

Which model head unit and what color wires did you connect the LOC to?

[Pound]

It's actually in a 2017 mustang. My son and his friend wired it up (my son's car) and they tapped into the rear deck speakers. I've never used a LOC before so i'm not familiar w/how they can be incorrectly wired. I know there are two pots on it, but they put it in a hard to reach spot and i'm not sure what the adjustment are for.

[smallfield]

If they tapped speakers (pleural) I’d bet polarity is reversed on one. For a mono amp you really just need to tap one, right? Disconnect or reverse one of the two speaker connections and see what happens.