The radio has 2 sources of power. 1 is the on off switch. a low power wire that runs from your gauge cluster which is why no gauge, no radio... This is why the radio shuts off when you turn the car off. The other is the power wire to keep all your presets and settings when you turn the radio off.
Its been a long day so those might be backwards, but I know one is a switched power wire with the ignition circuit, and the other is an always hot wire for the memory of the radio.