Upgrading factory stereo in 93 lx vert

I want to improve my factory sound system in my 93 mustang.. however, I do not want to remove the factory radio (unless best solution). I have no problem adding a secondary amplifier and upgrade the factory speakers. I suppose I can run an MP3 player and somehow switch the source of power to the amp when I want to play that.

Does anyone have any idea how to keep the factory radio in play, possibly upgrade the stock speakers and add an amp with switching input sources? Has anyone done this?

Thanks

A few ideas popped in my head as to how to complete this. One would be to simply use the factory radio and add an inline amp to new speakers. the improvement right there would be a good one. Now to use an mp3 player.. I would need to either use an FM transmitter and run it through my radio dialed in at the preset FM channel. Or make usre the amp I buy can handle a seperate input and simply choose that input when I want to listen to mp3's.

Not so difficult to achieve, just not sure of the cleanest way. I would keep the original speakers and use quick connects so that I do not touch the factory wiring.

Not sure I would want to take it as far as modify the factory radio.

Worse case, I simply use and aftermarket headunit and store the original for the future.