I have purchased a 2003 Mitsubishi Eclipse Spyder with the Infinity Stereo system.?

We have determined that the factory amplifier is not working..can I install an aftermarket amplifier and still keep the factory 6cd head unit? If so, is there some kind of harness I can buy to attach the factory wiring to my aftermarket amp? I have already removed factory amp and am looking to install my amp and subwoofer from my previous vehicle. Any help would be great!