I installed only an aftermarket head unit. No amp or woofer. The wiring harness for such an application should be widely available. At the time I purchased mine I think I had to look under Daewoo. Anyway, if you're using the stock radio and there are no rca jacks for the line out then I'm pretty sure your only option would be to use the speaker output. Hopefully your amp has inputs for the speaker output from the stock radio. That's about the extent of my knowledge on stereo installation. You can try this forum to try and get more expert help.
http://www.carstereo.com/forum
Good luck, Bill