the one that is labeled "-" or GND. Take a picture of your amp conections and i'll popint it out if you still don know

 

If everything powers on correctly (fan, light, etc), then the ground is probably not going to be the problem. The ground problem would cause the amp to come on and shut back down, play quietly and get hot quickly, or not even turn on at all

Here's a really stupid error, that I have actually seen people have... just checking to make sure. Is the fader on the CD player run all the way to the front speakers?

Also, make sure you check the connections at rear speaker wires to the (I'm assuming) line-level converter. Might just be a bad connection.

If you have an IPod or similar, you could try using a cable that plugs into the RCA connections on the amp and the headphone jack on the MP3 player. Turn the car on and see if you get sound. At least that will help you narrow down the area of the problem without having to fully swap the amp with a friends.

 

^ fader, high low inverter. Good thinking.