2000 Grand am rear speakers

I had to replace the rear speakers in my Grand Am last night. While taking the speakers out, there were 2 plugs connecting the speakers to the factory wiring, each plug consisting of 2 wires. I went and purchased a set of Pioneer 6x9 4way speakers that are supposed to be direct replacements for the stock speakers. The Pioneers only have leads for 2 wires total, not 4 as supplied by the factory set up. When I hook up the light blue and dark blue wires from one of the plugs, the speakers seem to work fine. If I try to hook up the other 2 wires from the other plug the sound quality diminishes. What I am trying to find out is what are the other 2 wires for and are they necessary for the Pioneers or are they just required of the stock speakers? I want to make sure that I am getting full use of the Pioneer 4 ways. Any help would be greatly appreciated.

  Hey Jeff, THANKS. That is exactly what I was looking for. I was just at the Pontiac dealer and they were looking at me like I was speaking Chinese to them. I am assuming that the light blue and the dark blue wires are the woofer wires as those are the ones giving me the sound I was after. Do you think I should use different(thicker) speaker wires or are the stock ones sufficient for what I have done?