As said, you can get an RCA converter which is fed by the rear speaker outputs, it then takes their signal and transfers it to an RCA output for you to run an AMP off, or 2 if ya use a splitter (they will recieve the same signal tho), Last i seen, JL Audio done 1 of these for around £75
Failing that, Kenwood make some AMPs that are fed by rear speaker outputs as opposed to an RCA, which is great for this kind of application. In saying that, they arent th emost powerful amps from memory, but are substantial for a medium powered boot build (1000w or so...)
Contact JL Audio and Kenwood, explain to them what it is you are looking, and they will point you in the right direction of where to get their products