2007 prius has aux port, but no disc/aux button

2007 prius has aux port, but no disc/aux button. The button only shows (Disc). I can't find out how to switch to aux mode. The owners manual only shows and talks about the disc/aux button. Anyone have any idea if there's an alternative way to get into aux mode?

We have tried cycling through the mode button on the steering wheel, and that doesn't work. And also, none of the buttons on the radio unit allows you to switch to aux mode. I'm wondering if the previous owner had the aux port installed and there's a secret to get into aux mode.