2004 grand prix monsoon

I changed 2 sets of speakers on a '04 grand prix this weekend.  I didn't check if there was a built in amplifed system (yes because I was overconfident).  By the way 2 subs were wired into the car previously by another install shop.  The customer was complaining about not enough bass and the rear-driver side speaker crackling.  He brought a couple sets of speakers to replace the factory ones.  By the time I was done, it sounded worse, no highs coming in from the rear speakers!  I fiddled around with the rears until I managed to get it going.  I took off one of the speaker wires to the subs and for whatever reason, the subs started to pound hard.  Does anyone have a diagram of how to properly wire up a sub to this car?  I realize I may be in the wrong forum.  Sorry in advance.