I took the cover off and pulled out the speaker in the driver's side door when I first bought the car, hoping there was a bad solder joint or something. All seemed to be as it should. It occurred to me this morning though, what if the speakers are wired in a fashion that if there's a problem with the first speaker of a channel, that problem would prevent the other speakers from working. I think the amplifier is in the radio itself (passive, rather than active crossover). So that may not be a valid line of thinking.
Anyway, I'm going to see how accessible the first speaker in that lineup is--the one mounted on the top of the dash, to see if that's a possibility.