you can hook up a amp to the stock radio, the radio does not have a pre amp out put, buy the rca line converter like sspecial mentioned, the remote wire from the amp, can be tap to the radio power antenna cable, even if your car don't have power antenna, your radio has a out put for it. an the rest is just running wires.
For the same price as the converter you can get a specific crossover for subs which has frequency adj. and bass bost for about 10 bucks off e-bay mine is a pyramid cr66.
It has rca or speaker level inputs.
Another company makes this part because about 5 places including radio shack/optimus have the same part with a diff logo.