RCA's with Bose factory system?

Anyone have any ideas about how to install an amp and subs in a 2001 Eldorado with Bose Premium sound and Onstar? I don't want to replace the factory head unit, I think its pretty smooth. Plus from what I have researched, if you replace the factory unit, even with harness adaptors, you will either lose your OnStar, or you will lose the chimes, both which work through the factory unit data bus.

I tried a line out converter, no luck, the bose system recognized that there were addtional speakers installed and made the inside sound absolutely flat. I've looked around and no one makes the harness adaptor for that car with RCA preouts.

The factory system sounds nice, but I miss my boogie box! :(

why not just take out your back speakers then put in 2 subs