I believe you have to get at them from underneath via the front. You can't drop them out from the trunk, and I don't think you can just remove the grilles and lift them out. Check the Interior/Exterior Mod forum. Someone had good pictures of the speakers and rear deck with the rear seats out. That was from back in August or earlier, so they might not be on the site anymore.
Don't be too concerned about removing the rear seat. You just pull up hard a little bit on either side of the floor hump to unclip it. Then the rear cushion lifts up and you can get to the bolts to remove the backs.