Yes it can! You use whats called a line out converter. Or you can buy an amp with speaker level inputs. Both of them take the signal from your speaker wires.
The tool is like 6 bucks @ autozone. Comes in a yellow package and is located by replacement antennae.just go to a local auto parts store...most will carry the removal tool.....find out what wiring is your rear speakers and just splice into those wires and you are money