How is pictured below.
And here is a link to why: Toggle Switch Wiring
You would need an SPDT switch to accomplish what you want.
By cutting the factory power wire to the radio and routing it to one pole on the switch and adding a new, fused circuit from your converter 12v side and running that to the other side of the switch, you will be able to select the circuit of preference (or turn the whole thing off if you use a 3 position switch). There are other factors such as radio lighting and clock/preset memory that must be considered. i. e. there could be issues that would make this complicated.