Stock Stereo Won't Play

Ok, I decided to sell my car, so I took out my aftermarket stereo.
After sticking in the stock stereo, the headunit wouldn't light when I put the headlights on. Instead it did exactly the opposite -- the display turned off.
So all of the sudden it just starts working like normal.
I still have another problem, though. I don't get any sound out of any of the speakers. I made sure the remote turn-on for the amp was working and I got voltage through at the other end of the harness (in the trunk), so I know that's not it.
Anyways, does anyone know of a way to check for an audio signal?
It's the only thing I can think of.
Either the amp went bad, or the audio signal is not coming through the wiring harness.

Thanks for the help guys.. :o

did you un plug the battery when you re-intalled? If not, you might have blown a fuse.

Nope, took it in to the dealership and they said I have a blown amp.
Thing is, they called while I was gone and said $800 to replace the damn amp. I went down there and they didn't mention crap about me paying.