[onewhippedpuppy]

When I’ve had that issue in vehicle audio installs it has been a poor or loose ground connection. I would gain access to the wiring and inverter, do a physical check of the wiring, and then start disconnecting components to narrow it down. Note that it could also be a loose connection at the speaker, so don’t forget that side of things. An internal issue in one of the components could also to blame. Does the hum change when you switch audio sources?