Sounds to me like ur gonna have to splice the wire from the outside speaker into the CD players speaker wires then you could just run the power wire to the power wire of the CD player,fuse box, or like you had said b4 a switch.
For this to work with music, you'd have a problem. However, I am assuming that the game call CD is not in stereo. You can simply splice the outdoor speaker into one of the inside speakers and use a switch to turn it on and off.
Depending on what you want to do, run a small amp, one you can use by tapping into your rear door speakers, and run the speaker off of the amp, while having a turn on(remote)Blue wire, when it comes to amps) Running off a switch. On/Off
Its easy to do, buy the stuff, and i could do it for you.
you could always buy a cheap, portable cd player, wire it into the electrical harness to get power to it and then run a speaker from that.
if you didnt want to listen to anything else while you had the calls going, you could wire and interupter or switch to all speakers and the switch would route the sound to your one speaker and disable all other speakers until you flipped the switch the other way. this would require a three way switch.
you might try finding the wiring schematic of any vehicle with surround sound and tv cuz my dad's yukon you could flip a switch to listen to the radio or wutever movie was playing. it was probably very similar to my previous statement.
Even better if you could find a way to hook it into ur iPod that way it doesnt play through the door speakers and you wouldnt have to splice wires, buy a switch, waste a CD to just burn calls on, and the iPod wouldnt get scratched up and skip like a CD that would be very cool if u fabbed it up that way.
Yeah its easy to wire, but you'll need alot of switches. You'll need basically 1 switch for each channel/speaker to turn them off. Not sure, but I'd bet radio shack would have a few switches that can do 2 speakers at once, but I doubt you'll find one that can do 4 at once.
Then you'll need another switch for the external speaker. The hardest part is chopping the stock wiring (not just tapping into the wire). But probably 10 bucks in switches and some wire and a cheap optimus speaker should do the trick.
We'll see if Bob or John have any more astute advice.