with that kind of mounting depth putting a box under the seat isn't going to happen.

only real option at this point is just to get some dedicated mids for the rear slots. i'll try to keep the price down but this stuff is still good quality

http://store.soundsolutionsaudio.com/products/pwx-6-5-crescendo-pro-audio-mid-range.html 2 of these 8 ohm models. these might be a tiny bit too deep but at most by 1/8 an inch so you can just rig something up if it is too deep.
http://www.sonicelectronix.com/item_43366_Rockford-Fosgate-Power-T165.html 1 set of these
http://www.sonicelectronix.com/item_22573_Kenwood-KAC-8405.html 1 of these
http://www.sonicelectronix.com/item_64092_Pioneer-DEH-X7500S.html head unit
http://www.sonicelectronix.com/item_4966_Scosche-SA68.html 2 sets of these
http://www.sonicelectronix.com/item_13217_The-InstallBay-AK8.html
http://www.sonicelectronix.com/item_68337_T-Spec-V6RCA-174.html use these instead of the rca that come with wiring kit. its a 2 channel kit and you need 4 channel rcas so this makes it easier.

optional things

http://www.sonicelectronix.com/item_20702_The-Install-Bay-VXT60.html good for protecting them from water dirt on the back side. these might also solve the mounting depth issues on the rears.