If the entire radio was not working then you could check for a blown fuse. Have you checked the driver rear door is that working?
You can check the speaker with an ohm meter by pulling the door panel off, taking the speaker out, and unplugging it. With the digital volt ohm meter put it on the lowest ohm setting and check the speaker for resistance. The speaker should have the ohm number on it. If your reading is anything other than that number you have a bad speaker. Also check the speaker for any torn or broken spots.
Another way without any special tools is pulling the passenger side door panel off along with the drivers door pannel and switching the speakers to see if the problem follows the speaker it will also check the drivers side wires to see if that is the problem.
The panels are fairly easy to remove.
here is a video of the process.
