Definitely one of two things:
Most likely the problem is that your positive and negative speaker wires are either touching, or reversed. Try switching them around to see if you get a better sound. If not, try using electrical tape around where they are connecting to the speaker and make sure there are no frayed wires splitting off and touching anywhere on the speaker or surrounding areas.
The second, less likely but still possible problem is your ground wire is bad. Try finding a spot on your car that is BARE METAL. This seems easy but can be difficult. If you can't find a good ground spot, try using a multimeter to test and find a good ground spot.
These are simple solutions that can easily be done at home. I would say go to Circuit City but those jack asses will probably just screw it up some more.
