Thanks so far guys. No solution is ever perfect.
I have used an iPod and ear pieces for while and find that a little uncomfortable. Audio streaming appeals, but is never as good as an USB input I am told. However, newer cars seem to almost universally fit these systems. Jury is out on that I guess.
Speakers are another issue and ones behind the seats are least invasive but depth is an issue. What kinds of speakers have been used successfully?
With smaller audio systems that can be hidden (I am not thinking about an in-dash setup) is an amp needed? Are there suitable small amps?
Last, radio is useful at times so that is another factor.
Has anyone used these? Goes under seat and in glove box:
Secret Audio SST iPod/USB/MP3 Classic/Collector/Muscle Car Stereo AM FM Radio | eBay