ok since i did not have the infinity sound sys in my car im not sure what they did to wire that. baicly i can point you in a do it yourself route which ill explain everything even better than i can for about 10 bucks at
www.crutchfield.com that is where i got all of my stereo equipment and they even send a very nice booklet with your car and where and how to take it apart and put it all together stereo wise they also have great customer support you can call and they will deff help.
Now for what i can do for you is to tell you to check the wiring to your head unit and see whats going on there, if all speakers are playing then the amp was not wired right to begine with and the head unit is sending the signal and power to the speakers. If it is a aftermarket head unit its a simple fix the harness for your wires behind the head unit are hooked up therefore the speakers wires in the harness behind the stock head unit (which since this was the premium sound package the stocker probably had that deleted on its harness and sent signal just to an amp) is now sending the signal and some power to the speakers too. Clip the wires and set it all up so that only your amp sends the power out and the head unit only sends signal to the amp.
A GREAT(albeit messy and hard work) fix is to get a pre out headunit......if you have no subs a2 preout will be fine with subs a 3 preout with one pre out specifically set for the subs (this eleminates all high frequency signals to the subs) is what you want at this point you want speaker wires and signal wires, you want connectors fuses and power lines...once again refer to curtchfield.com they will tell you what you need depending on your power (so you get a guage bigg enough to handle the load) and what not, plus if you spend like 200 or something with them they send it free and i got a year of car and driver for free from them too.
seriously though it would be a great idea to check crutchfield, they will even tell you what fits in the car, andlike i said send you a wonderful pamphlet full of informaon that you will need to get tihs all working well.