the rear speakers are subwoofers, so there wouldn't be any sound coming out, but there would be vibration. So if there is no vibration like you said, you could have blown them out from playing the tunes to loud, but that probabaly isnt the case. They might have just died, it is a 6 year old car, things happen. But just make sure again, that there is no thumping or vibration from the subs hitting first. Otherwise order some new subs because they probably went. If the music is playing out of the door speakers, that eliminates it being an amplifier issue I'm pretty sure and therefore the subs would be the only problem