2002 impala ls, subs to stock stereo

I am trying to install hook up subwoofers to my girlfriends stock stereo system. I spliced the RCA cables into the right wires, and the power is hooked up properly, but when I plug the RCAs into the amp, it goes into protection mode. I don't understand why it is doing this because the amp and subs are good. I don't have them hooked up to the factory-amp outs either. I have them pretty much directly hooked to the back of the radio. I need some suggestions or a fully descriptive wiring diagram for this car. Any feedback is appriciated.

Did you use a line-output convertor?  If you didn't you need to get one because you can't take rca's and splice directly into the wire.

Really. That was very helpful. I will have to try that. Thanks for the help. I'll keep you posted.