I haven't done it yet, but if it were me (I don't know what model car you have)
I would just mount a two channel (two speaker) amplifier somewhere hidden in the trunk for a coupe, or under the front seat for a sedan and then run a mini-plug to RCA cable to the input of the amplifier and use an iPod or iPhone as your music source.
You can now stream like an internet radio any genre of music on the iPhone, and when you are at a show, you can unplug the iPod sitting next to you on the seat, stuff the mini-plug cord down in the seat and voila, no radio in plain view.
I would mount the speakers in the kick panels of the car, but another thought I've had is putting car speakers inside drive-in movie speaker housings and wiring another mini-plug end onto the cords. Then just mount female mini-plug jacks somewhere near the kick panels and when you want to drive and listen to music, hang your drive in speakers on the inside of the doors and plug them into the car. Again, when you stop and want to show, you can unplug the speakers and hide them in the trunk. No one sees your mini-plug jacks under the gas tank and it looks old and stock.