I'm not sure about your '04 Grand Prix, but I have a 2001 and I recently had to take out the back seats in order to mess with the speakers in the package deck. First order of buisness is to take the bottom off. There are two little cables on the front of the seats, directly center and right under the front of the back left and right seat. find the small ring to place your finger, and simply pull, after bulling both the bottom comes off like cake. The back-rest part is the most trouble I've gone through in this ENTIRE speaker process. I found it a lot of help to lower the middle entrance thing to the trunk (The little hatch door that on a 2001 requires the key to opwn and allows access to the trunk area from I guess what you would call the cab. And for future purposes I'll reffer to it as an access door I guess.). There are four bolts that are 11mm in size, and I used a wratchet wrench thing to get them off. Two on the left, and two to the right of the access door. The first pair are located about 6 inches beneath the top of the opening of the access door. acter removing them, move to the pair about 1'-6" beneath that set. They are about 4-6" beneath the bottom of the access door and just to the left and right.(Don't worry about these four screws falling, as they will only deposit themselves on the area under the back seat that is now revealed). OH! And by the way, don't know if I mentioned it or not, but the bolt heards are located not in the cab, but are in the trunk drilling into the cab. After manuvering the wrench and your hand around to take the bolts out, the seat is almost free. There are now two smaller bolts (4mm I believe) that are directly beneath the back-rests, and are directly in the middle where a person on the left or right would sit. If you were sitting next to the windows on either side, follow where your spine would be down and you should find the bolts, which are on the inside of the cab. Now after the 6 bolts have been removed, lift up on the seat and it should come out.

I just did this 10 minutes ago, so it should be accurate enough. However, if my memory fails me, or theres an easier way to go about it, please feel free to add anyhting or make any changes.

Hope This helped.