What kind of speakers are the stock rear speakers? Crutchfield says they are 6 3/4" speakers, but from the service guide it looks like they are wired with both a + and - signal pair AND a voltage/ground pair. Do they have some kind of amplifier built into them? Are there any after market speakers that would fit here?

If I were to install a new head unit, would it be as simple as bypassing the factory amp and sending the rear speaker outputs on the new head unit to the new rear speakers?

I understand that the factory amp still needs to be there even if you aren't using it any more because of the other sensors that are on board.