Is there a way to go from driving-the-car power On mode, to just Accessory mode, without turning off the car? I wish there was.

If I have music buffered in my Sirius tuner, it goes away when the car shuts off, but if I want to park and continue listening, do I need to remain in On mode? I'd love to go to accessory mode, but I don't see any obvious way to do it.