How do I open rear passenger door

Could be the child lock switch -left rear door working?

Left rear does work normally

You might be probably right on the child lock-if not check the master control switch

Where will the master control switch be located? Since the door is in closed position, I can only undo the screw in the latch and in the arm rest.

Its the master switch at the driver's door-that control the windows/doors operation

OK, I did check this already. Central locking working fine. I think when I removed the inside door panel to access the speaker, I twisted the door latch to get it through the opening and in the process might have pulled one of the "wire rods" from the latch to the opening mechanism out of position.

I call it a latch, but what I mean is actually the lever to open the door. It seems as if there is no resistance at all.