To do what you specifically ask (speaker level to RCA) you will need an adaptor like this one...
http://caraudiosecurity.com/shop/pro...s_id/3794.html
I am sure many other companies make these. The best ones have varible output (so you can match the proper optimum voltage for your amp)....
If you want something more sophisticated (and more money) .... this is the active device that will do the same thing:....
JL Audio: Products
personally - IMHO - if you have the $$ - clean sweep is the way to go.....