The stock infotainment source has to be on the "aux" input and then you press and hold the nav physical button on the dash for 2+ seconds to get it to toggle back and forth between the naviplus unit and the stock audio.
To get to CarPlay, I don’t need to hold for two seconds, I just tap the any of the top few buttons on the left side of the screen (Map, Dest, or Menu). On the lower controls, I have Disc-Aux. When I had the old Vais Bluetooth connected, hitting Disc-Aux would cycle through the disc and Bluetooth. With Vais out of the equation, hitting Disc-Aux only plays the CD, doesn’t switch over to the new aux source, CarPlay.