A line level adapter. You'll need to splice it in/sauder (sp?) a line level adapter to your rear spearkers, that will give you the RCA output or whatever you'll need. I think I got one for $30 or something at Quality Auto Sound. Probably way cheaper on Ebay. But that should work to wire in subs and amps.
PM me if you have questions. Other than a new HU, there's no way to do this unless your amp has a speaker wire input thingy.