my 1999 grand prix gt rear 6x9 dont play but the 4in beside it plays whats wroung

That stereo is a 9 speaker system I believe. The two rear speakers are combo mid-range and subwoofers; door speakers are full range, and one center speaker in the top of the dash. At any rate, if the speaker can be made to play when hooked to another unit, then there's either a broken wire somewhere, or the unit in the car has gone bad. If all the other speakers are working ok, I would assume a wire is broken somewhere between the radio and that speaker, and go from there. The easy (well not so easy) way is to just run a temporary jumper pair from the radio to the speaker, following the same color wires of course. If the speaker works, you have a broken wire. If it doesn't, you have a bad radio.