How to make a VOX for my car to switch pre-amp level audio. Want to switch out my iPod for the GPS when the GPS "spits out" directions?

On the heals of my last question I want to automatically switch audio sources in my car using a VOX like feature. I would like to make it a smooth switch (no pops). Where I found out about some really nice chips which can do all the work.

I would like to finish up by asking how to go about designing a VOX to control the audio multiplexer chip. The inputs to the VOX / audio multiplexer will be the Line Out of the GPS and the iPod adaptor. The output will be to the car's radio head.

All the electronics are in place. What is missing when listening to the iPod is a way to autonomously switch from the iPod to the GPS when the GPS is giving out directions. Garmin GPSs are nice about this (I assume) as there is a bell sound before any directions are given out.

So the VOX has plenty of time to switch before the directions are given. For those who are thinking this - yes, I know most if not all Garmin's have methods to play back music and nicely, on their own, switch between that and directions. But sometimes what I want to listen to what is on the iPod.

Besides, the iPod is already nicely integrated (text comes up on the radio) into the car.