I just finished installing a JVC MP3 player w/ 12 disk CD changer in my 94 SLS that came equiped with a CD/Cassette Bose system. The CD section on this radio died, so for a year it was either cassette or radio. I didnt want to change it because of all the changing to the system.
Finally took the plunge, bought a mounting kit (which required a lot of modification to fit properly. It might not have been the exact one.)The JVC was a little deeper than the stock unit, so it took a lot of tweaking to get it to fit just right. Then I also had to file the trim so that it would fit back on. It was not fun and I spent a full day on it.

I ran all new RCA jacks,(didnt use the JVC speaker outputs at all) and connected them to a Kenwood amp, capable of two Ohm loads. I ran them along the passenger door sill.
I simply unplugged the BOSE amps and hooked up the factory speakers in series directly to the Kenwood Amp.
I used the factory wiring for the two door speakers, and also hooked them up in series. I have to admit that I thought it would only make a small difference in sound,, but it actually makes quite a difference.Later on I will listen to 6x9's at the store, and upgrade.
It was not an easy install, since I had no wiring diagram and only a meter. Actually I only used the factory wiring for power and ground at HU, front door speakers, and pwer antenna. Everything else was left intact, so if I was to put factory stereo back in it would be to simply plug in everything and a few splices.(I dont know why I would?)

Anyway, that was my experience, hope it gives someone an idea of what could be done.