rear 6x8's not playing....out of ideas

I would first start with the obvious such as make sure the wire wasn't flipped on the speaker connection itself. It sounds stupid but I have seen it done. Second bet would be to pull out the player and check all the wire connections. If you don't have any luck doing that you might have a short in your wires to the rear. So while your headunit is still out take some audio wire and run it to the red and white connections on the back of your player (haven't seen the back of stock, but all of the custom setups I have done have it on there), then see if that works. If it does you have a short in the stock wiring and you need to replace it all.

you could also try unplugging your front speakers from the back of the headunit and plugging them into the rear channels. it could be the headunit

Sounds like you've blown the rear amplifer outputs or lost the rear channel signal. Is the system after market or stock? If it's aftermarket, are you using the speaker outputs of the head unit or using the line level outputs to dtrive a separate amplifier?

Speakers rarely just melt down to the point they don't play - they usually start sounding aweful as the coils begin to start scrapping the magnet. If the system is stock, it's unlikley even at full volume that the speakers would just die - especially at the same time. It all points to something upstream.