Check whether your stock headunit has RCA ins (most likely it won't) - without any RCA ins you cannot hook up your amp to the headunit. You will need to get a converter/FM modulator like fr3aki3 said, which converts the speaker cables into RCAs. That connects to your amp and the rest is just like usual.

Bear in mind that with this setup the sound is coming from a frequency from your radio and hence quality will not be optimal.