Found it rather quickly using the tool recommended by @mlpeloquin and described by @centerline . Thanks! It's hilarious. First of all, I don't give a rats body part about the external speakers having red LED backlighting. Second of all, why in the world would that external light be on the same switch as the one that controls the indirect lighting above the slide? Anyway, that's the offending shorted wire, the one to those speakers. That wire is now cut in half, capped off on both ends, labeled well, the two ends taped near one another for some future owner that cares, then dropped back inside the wall behind the switch.