Assuming that the Balance and Fader controls are set correctly, you have done the other proper diagnostic step by inserting another car player.
By inserting another car player, you have ruled out the wiring and the sled. Good job.
Check to make sure there's not a piece of crud on any of the tabs of the docking connector, and make sure your player is fully inserting into the sled.
Also, try pressing on the front of the player and see if you can make the speaker come in intermittently. If so, check
this FAQ entry.
If pressing on the player doesn't make the dead speaker come in, it might be a loose wire inside the player or some other fault. Contact support and see what they want you to do.