The question you ask is too involved for a proper answer. You may want to go to a car parts store, and get a repair manual for your car. They cost about $16.00 Or, go to a Public Library.
That really depends on the kind of car you have, but you should be able to go in through the rear armrests and plug the rear speakers into the old front speakers. Most people have speakers in both the front and back of their cars. The order of installation is not important, just have it done the most convenient way for you. You could even omit the front speakers entirely if you wished. Electronically it does not matter.
You remove the clips on the top, screws by the fenders, remove foglights if equiped and remove the bumper, install the new one in the opposite order.
Lift the front of the vehicle up and support it with jack stands under the lower a-frames. Remove front wheels. Unbolt front shocks and remove them. Reverse order to install.
Remove tire, install spare which has an open center hole allowing access to axle nut, remove axle nut (33mm socket), remove spare, remove caliper, remove rotor, remove hub/bearing assembly, install new hub/bearing assembly, and so on in reverse order.
you have to remove the whole door panel in order to gain access to the factory speakers in which should be 4x6.
In order to remove the screws that secure the rear speakers, you need a 7/32" socket and socket wrench. The wiring harness I have yet to figure out how to remove from the backs of the speakers lol.
You have to remove the door panel. After you will see the speaker which sits in 4 screws. Remove the screws disconnect the speaker and in the reverse order for your new speaker. Keep in mind, whatever new speakers you get, the stock amp on this vehicle has a low pass filter to the door speakers (and a high pass filter to the dash) so they won't deliver any highs.
In order to replace the speakers you have to flip down the rear seats and look at the screw ports and remove all the screws to remove the panel covering the speakers. The rest is self explanatory.
6 1/2 inch for the front doors and 6x9 for the rear deck. You HAVE to remove the back decklid in order for the speakers to come out. They are dropped in from the top. You can also go to Crutchfield.com to get the specs for your car and the wiring codes.
Remove the wheel, remove the brake caliper, remove the brake rotor, disconnect wheel speed sensor from harness. remove the bearing hub, remove the wheel speed sensor from the bearing hub. Reverse order for install.
You are going to be shocked, but you actually have to remove the ENTIRE back seat in order to get to the rear speakers. Try to use the Nissan speaker adapters plugs so you don't need to cut wires and mess up the electric system, you can get those from eBay or amazon.
There are 2 coil packs, one on each side of the engine toward the front. Look for the spark plug wires and what they plug into at the front of the engine is the coil pack. Remove wires making note of which goes where; remove wire harness; remove screws holding old pack in place. Reverse order of removal to install new pack.