Why won't the stereo work?

I was installing an aftermarket cd player in my friend's 2001 Dodge Stratus and when everything was done the stereo didn't work...so I tried my brand new stereo...same thing, nothing worked. so we installed the stock stereo again and guess what? NOTHING!! Please help!! I checked the fuses for the radio and they were both fine, the only thing that does work is the buttons light up on the stock stereo.