Typical mobile players detach from a cage by inserting a removal tool through the faceplate. Check with your mfg to get detailed instructions.
To diagnose the problem, you would need to know if there is any signal voltage from the player to the speaker location. That signal will be low AC voltage. To trouble shoot the condition of the speakers you could measure the resistance across the unconnected speaker terminals, it should be in a range around 4 ohms. To identify if the signal is missing from that single location, you could jumper leads from the existing, functioning channels to the non-functioning speaker locations.
Make sure you don't have a button or menu that turns on and off a channel. It could be that simple.