I just recently got a 2004 GP GT2 and the driver side rear deck speaker is not playing any sound at all. Even when faded to just that speaker there is no sound. Is it blown or do I need new wiring or what?
Try swapping the right side speaker with the left side. If yours is a Monsoon system, there's a bit of wire there to play with, but I'm not sure there's enough without pulling the speakers.
If you have to pull the speakers, you need to pull both C pillar trim panels, (one screw each under the airbag emblem, then clips) the rear seat bottom, (sort of a "T" handle cable under the left and right sides about a quarter of the way in from the outside of the seat) the seat back, (four 10mm socket bolts along the top, three 18mm nuts at the bottom, including the seat belt buckles, and two 18mm socket bolts on the outer ends of the bottom edge.) Snake the three seat belts through the package shelf trim, then pull the trim out. The speakers are held in by two clips.
If you have the Monsoon system, there will be two plugs for each speaker. One is for the "sub," the other for the mid range speaker. Make sure you don't mix them up. If I recall, the solid color wires are for the mid range, the wires with the tracers are for the subs.
Since each speaker unit contains two speakers, it's entirely possible that the speaker unit itself is bad, but I wonder how likely it is. The odds of both speakers within the unit being bad aren't really all that good. As I understand the system, the rear channel mid range are separate channels with the higher frequency sound sent to each speaker by the built in crossover in the amp. The "subs" are a single output from the amp split to drive each speaker. This paragraph presupposes a Monsoon system. That's easy to determine. There's a center speaker in the dash that says "Monsoon."
If swapping the speakers fixes nothing - you've got nothing to lose but about a half hour or so of your time removing and reinstalling the rear seat - I'd be inclined to believe the head unit in the dash is bad before I'd look at the amp.