I've done the almost the same thing with a radio shack project box and a cheap radio shack DPDT pc board relay.
I'm running the empeg audio into my stock head unit through the cd changer input. I routed the cd changer adapter cable into the project box and setup the circuits so when the empeg antenna wire is energized the cd changer output is switched to the empeg aux input and the empeg output is switched to the head unit cd changer input. When the empeg is off or removed, the relay switches the changer back to the head unit. I've had no problems with noise or any detectable loss of sound quality.