Sounds like you know what the problem is, your left channel isn't properly connected to your speaker. Most likely a wire problem or a stereo problem. Is the stereo properly grounded? did you check it?
You may be able to check the speaker wire, though that could be kinda hard because of how it runs.
What I would do is disconnect that channel from the current speaker wires, then I would hook up another set of speaker wires to that channel and run it to the speaker in question. you don't have to run it under stuff, just across the seats for testing. If it works, then you know the problem is with the wiring. If not, then it not wiring, you need to look at the installation of the stereo.