Unless your just looking to just get some basic tunes in your truck, putting speakers in the kick panels will give you the best sound out of any location in your vehicle. Putting full-range speakers under or behind your seat aims them towards the floor boards or back of the seat(away from your ears) and muffles the sound.
Assuming there is enough room and whatnot...if I were putting together a system, here's what I'd do:
5 1/4 or 6 1/2" (whichever will fit) components in the kick-panels
6x9's ( or largest that will fit) in upper rear corners for rear fill
8 or 10" sub(s) under or behind seat
Here's some universal Q-Forms kick-panel enclosures, if ya dont wanna build your own. Kinda pricey if you get em from Crutchfiled, but you should be able to find them cheaper if you shop around a bit.
http://www.crutchfield.com/S-69obkaP...search=q+forms