Aux Input - no sound

First day with a 2006 Prius. I've selected Aux on the stereo, (stereo is on) plugged my cable from my iPhone 8 into the Aux in, but the music only plays out of the iPhone speaker?
I'm using a lightning splitter, where 1 side is for a charger cable and the other is for audio (so you can charge and use the headphone / audio out at the same time) I tried turning the stereo off and on again, but still no sound from the stereo system. Do these Aux input jacks go bad after some time?