2002 Kia Optima New Stereo not working

[boomboom]

I just installed a new Dual stereo in my 2002 Kia Optima. Everything appears to be working, but no sound is coming out of the speakers. I used an aftermarket plug and traced the wiring. The power antenna works. The CD comes on. I hooked up an extra speaker to the wires past the plug and I get sound. When I plug in the stock stereo everything works. Is there a stock amp or something that needs to be turned on?

Thanks.