[dogslovetrucks]

Fronts are easy- 4x6 fit in the door w/no problem. You generally have to go with a more shallow mount spkr in the doors if you have power windows (most people do). With that said, Polk and Kenwood both offer nice shallow mount 4x6 that sound nice and are pretty cheap ($100 or less for the pair).
The rears are a different story- Very difficult to replace and actually involves removing the rear glass side lites to get to the speaker housing. I think there are a few posts about this subject and may be even a few that give non-factory "alternatives" to getting to them.
Many use the rear storage compartments to mount 6x9s-