From what I can tell, this gentleman modified the original stereo to allow for a phone to connect to and play music, plus added some amps. May be a good solution for some.
https://www.youtube.com/watch?v=wVc9svXp8Nc
I have been wanting to leave my original radio in the truck and then just build a small amp under the dash that runs to the speakers that I can jack my phone into. I just thought of something though, they should make bluetooth enabled 6 by 9's that are wired for 12 volts, that would make everything else moot. Oh well, have a look.