I did the same as dabumblebee, made a shallow box and put 2x10's in it. my seats are the fold up kind (not sure if that is what is considered "hideaway" as they are just folded against the rear of the cab)
the seats wont go down though, the speakers I had are too thick so I had to make a box big enough.
but with a certain type of shallow speaker you can do it right, where the seat would still be able to fold down. not sure who you could possibly fit back there though with the speakers in place, as you would have zero legroom.
my stereo is one of my favorite things about my truck, it rocks, even with stock speakers in doors. which I'll be changing out here pretty soon (they arent blown, I just want better sound from them)
looking to go as large as I can in the rear doors, and stay with standard size in the front, and will be adding 2 tweeters on the front dash area. (maybe 2 more in the rear area, but not sure about that yet)
Ive also noticed that if you dont use the 2 storage compartments in the rear doors (I use mine), you can fit a decent sized speaker there, possibly a small sub speaker, then you could forego the enclosure.