The Grand Prix could have a wiring problem. Check the wires to the back speakers for any loose connections or shorts. If the wiring is on then the stereo probably has an internal problem.
It is most likely your front tires and then that woul mean your wheel bearing.
I bought '01 GT rebuilt with no stereo or speakers. Installed Sony xplod all around .I used rear panel holding speakers and existing speaker covers to mount 6x9's.(cut out oval using hack saw) speakers look real good and sound nice. also used same technique to mount 3 way, 5" xplods in front door panels.
broken transmission mount there is a front left, rear left and a center mount
If the receiver is just stereo, not surround sound, you will not be able to create a surround sound (front, centre, sub, surround) setup.
The Pontiac Montana does not come with a factory installed amp in the sound system. The wiring goes directly from the radio to the speakers.
Ideally, the system should have the same brand and model in the surround as the three front speakers .
5.25" front and rear sound bar
sounds loke the wheel bearings are going out
Depending on if you have the Monsoon sound system or not, it may differ. If you don't have Monsoon: Front Doors: 6 1/2" & 1" Tweeters Rear Deck: 6 x 9 I'm not sure what is in a Monsoon system, I do not have one, but I hear that it has 8 speakers throughout the car.
You are overloading the factory speakers.
definately sounds like an electrical problem probably the altenator.
sounds like a bad ground