I was thinking about going with the Whelen product. It is definitely going to be a costlier route, but my buddies wrangler has the Cobra CB w/ PA system and its way too quite for me. Plus always have to 1-up the wrangler.
http://www.whelen.com/_AUTOMOTIVE/de...68&prod_id=187
Its got a good detail about it:
http://www.whelen.com/install/133/13327.pdf
Seems like it may be a way to go and then just install the Onstar CB without a Pa like HumminNBoatin did. I wont use the CB other than listening except when on a trail or roadtrips. So I can just leave the CB microphone detached and have the Whelen attached and visible. Or the entire Whelen system in the center console and the CB microphone exposed