Switch/Wiring Question: Stereo Ignition Power Override
I have a sony head unit I would like to be able to power on even when the car's ignition is off. The unit remains powered when the car is off, in order to preserve its memory and in order to be able to load and eject discs but it will not power up and produce music while the ignition is in the off position. I would like to be able to temporarily flip a switch in order to override this feature when I want to listen to tunes when the car is off, but retain the function of powering down the stereo when the car is turned off in normal circumstances.
The stereo is fed by two power (12v+) wires, one being a constant power line from the battery and the other being ignition switched. If I have a simple Single-Pole-Single-Throw on/off switch, could I splice the constant battery power connection into two, running one wire to its usual position to the constant power lead on the head unit and the other wire through the switch and then into the wire on the head unit designated for ignition switched power (note that the original ignition switched wire would still be connected).
My only worry is that when the switch is activated, it would feed always-on 12v+ power into the car's ignition switched line. Will this hurt the car? Would I need to upgrade to a single-pole-double-throw switch where the ignition would be broken off the circuit when the always on 12v+ is applied?