I have an setup on my coupe that I like, as it can be completely removed (permanently or just from sight at a show or cruise) without much hassle.
Under the seat I have an inverter (on a switch) that provides 12v power to a tiny 400w amplifier. The amp I got for $5 (including shipping!) off ebay.
The amp powers one of those "portable speakers" that they sell to provide better sound for CBs and FRS radios. The speaker sits on the package tray. This isn't the exact speaker I have, but pretty similar:
The wires to the speaker run up beside the seat.
I have an input wire that comes up between the seat back and cushion that plugs into my iPhone which I'm using as an MP3 player in the car. I set the phone on the seat next to me--where I can easily get to it if I want to change songs or adjust the volume.
If I want to hide the music away, I unplug my phone, open the seat and tuck the input wire and the speaker (after rolling up the wire) down inside. When the seat's closed again you can't tell that it was ever there.
The sound isn't great (I only know because I've listened to it with the engine off--while the car's running the engine noise and various Model A squeaks and rattles hide any distortion) but since I'm mostly playing 30's tunes while motoring along it's fine for me. Heck, even if there's some audible distortion, how would I tell that from the record pops and clicks and tinny sound of the originals?
If you want CD quality sound you'll have to do something more expensive/drastic.
HTH.