Yes, I have the same problem with my '06. I bought it last summer and didn't notice a problem at first, then the left side became intermittent but, as with yours, the tweeter works. It is no longer intermittent... it just never works.
I do not suspect the head unit because it also affects the chimes. The signal light click works but I think it plays through the tweeter. The driver's seat belt, key in, and lights on chimes are barely audible through the tweeter. The passenger's seat belt chime plays through the passenger side speakers, and it works fine.
I suspected that an aftermarket stereo had been installed at one time, then the wiring not put back correctly when the OEM radio was reinstalled, but I pulled it out last weekend and the original radio does not appear to have ever been out. The wiring looks original, with no signs of splicing.
I suspect that there's a module under the dash somewhere that interfaces the stereo to the various chimes. If this actually goes through the head unit, then maybe that is the problem. If they come from somewhere else and do not pass through the head unit, then the problem is downstream. I do not think that they go through the head unit, though, because I know you can put an aftermarket stereo in with the appropriate adapter and everything still works.