If you have a factory amp, it will be under the passenger seat.
honestly dont know about the harness if you have an amp, when i did my install, i ran all new wiring to the speakers, and all new power wiring as for competitions, i cant afford to have the voltage loss through the cars factory wiring setups.
If you are competent with wiring in a stereo, and want to save money, then this is not hard.
From memory (i replaced mine 2 1/2 years ago) the speakers are 6 1/4" in the doors, but dont quote me on that, they could be 6"s, i cant remember exactly, and the rears are 6x9's