When I play a cd in my car, on the drivers side, You can't hear the back speaker at all, and the front speaker sounds icky, like it's been blown out. The right hand side is fine, and when I just play the radio, it's fine.
If the radio and cd player are a single unit then the culprit will be in the "reader" portion of the cd player. Don't they sell cd player cleaners? If they are seperate units, then the cd player will plug into the radio which in turn will go to the speakers. The easiest thing to do here would be to unplug the connectors, clean them with contact cleaner and then reseat them. A high end factory unit might utilize an amp, but that doesn't matter. It sounds like the problem is between the cd player and the radio.