Where are you putting them? Mounting them will be a problem if there aren't pre-fabbed areas in the car for speakers, and then wiring them will be another problem if you are adding onto 4 speakers to get 6. Unless your head unit (CD player/radio) has connections for 2 additional speakers, then you are going to have to figure out a way to splice some connections without losing audio quality to the speakers on spliced lines. (Amp probably)
Running the wiring, if it needs to be run, is the easiest part.
-SW