91 Stereo????

I recently bought a 91 lx and it came with an aftermarket cd player. i was gonna put the stock radio/cassette deck back in it. when i did it had power but no sound....then i saw another plug that was buried in the back of the stereo area......this plug would not reach the stereo but i was able to plug it into the radio by pulling the plug around some other wires....anyway it worked......my question is how is the stock stereo plugged in? is the plug in the very back used? and finally the two plugs in front, one grey one black....the grey one is used but the black one isnt...is that correct??

thanks...sorry this is so confusing but any help is greatly appreciated.