Do you have a navigation system?
I thought my left front speaker blew too then, techie brilliant genius that I am (still can't program a VCR) realized the navigation system voice shuts off the left speaker when she talks. I then discovered that I didn't have her turned up loud enough so it seemed like the speaker was going out, it was just the crazy navigation woman interrupting to give directions. I also realized if you crank the tunes, no matter how loud the navigation system is turned on, you can't hear her very well when she butts in.
I suppose if I just sat down and read the manual I'd be better off but I haven't had time for it and I can do the basics and download my favorite tunes so for now that will have to do.