OK, I did a little more research and found that some CBs have a PA switch
and I guess that's what I'm looking for... but then I need to know... could
I plug that into the AUX input of a car stereo, or will it only work going
directly to its own speaker?


They are designed to drive a speaker directly. A cheap powerhorn from
Radio Shack and a 1/8" plug will have you in business. All you would
have to do is mount the PA horn somewhere pointing toward the riders.