How to use aux with no input

You can't. "Aux" means auxiliary input for your stereo system. So the radio is looking for you to plug a headphone jack into it, which is usually connected to a cellphone, laptop, tablet, CD player, or even cassette walkman if you want to go back in time.

Only plug in a device like ipod, never head set. If you plug in a tablet, be sure to plug the output of the tablet to the input of aux, Plug in the wrong thing and you could blow it.