If your tied to the courtesy lights, it is not direct from the battery, as Yamaha loops their power and jumpers it from one system to another on a single home run, except for accessories. So using them as a power source could be your issue. I ran a power supply cable to a separate switch panel. Mine didn't hum, so I can only offer what I have said. I agree with
@Bruce , you can't isolate the ground, but you can isolate the power supply. And you can also install a ground loop isolator, a cheap fix and might help. They are typically used to reduce engine noise in the sound system. But as stated, all grounds are essentially connected, so it might help.