Loose connections between radio and left side speakers. Or something in the stock headunit is fried. I just got threw re wiring my entire aftermarket amp because it stopped working, still didn't fix it, took the amp to a audio store turned out water/moisture fried it. Even though this has nothing to do with your issue, electronics are tricky. It could be a lot of issues I am going to assume that there is a short in the radio somewhere between the radio and speakers.
Try checking the wires out from the head unit to speakers and make sure there all connected.
I can tell you though it isn't the speakers
since there new lol.
I'm not to sure about the fuses because I am pretty sure if it was a fuse the whole radio wouldn't work. Not just one side.