I am not totally sure about the Gen II's but am pretty dang sure they are 5X7/6X8 like the Gen 1's which are the same overall size, one just has a little more woofer area. Personally I would go with a set that has multi directional tweeters and sounds warm with good human voice "punch" through the music with good bass guitar (not thumpin bass) unless its only rap than same applies just with a tight low range response as well. I am not aware of stock rear deck speakers on the Mark VIII's though I have never really looked at Gen II's.
All you nead is like a smaller flathead screwdriver and a torque head (sometimes called a "star" due to its looks) screwdriver to remove the speakers. Slip the flathead under the bottom rear grill and gently push away from your body and it should start to pop right up and go along the bottom and gently do this until you feel its ok it gently remove it, make sure you pull down and clear the top 2 vertical clips before pulling out. Torque drive will remove the 4 screws holding the speaker in place and make sure you get the aftermarket harness' for the speakers so you don't have to hack up your wires to install aftermarket.
Up front is where you want the guitar, cymbals, the meat and higher frequency stuff, etc to come from. It has to do with the human hearing "setup" and the doppler effect. Personally I wouldn't try cutting anything or trying to make more room UNLESS you have massive money to have a REAL PRO do it. I looked at stuff like that and its not worth it; and music is my life.
Best bet is the above with like a kenwood 4 channel KAC-6402 (I only say that since they don't have the same series I have anymore, which kicks ass) or something with built in crossover and variable freq/level EQ that has enough RMS~~~~very important~~~~ power to match speakers for max results. Plus you can mount the amp in the trunk on the backseat cross-member to keep out of the way. A good deck makes a big difference too.
I know its long but I have a lot of experience with audio and this car. Any questions or need help or an opinion feel free to ask.
I took a few photos just for you, well.... ok to show off a little too
#4 you can see how the tweet is aimed forward and up towards the persons ears.
#5 is a 4 way speaker to get crisp, deep, rich mid and treble.
#6 is my Kenwood dvd Head Unit closed, #7 open with audio on-screen.
#8 is an overview of my trunk with Soundstream Tarant. 1600/2 (Chrome, yes real chrome) and custom green glowing LED I made and installed, green spider eyes and circle from under center of tarantula badge and from inside amps cover vents, complimented with blue LED in trunk lights and interior except map lights, my handmade cover on the left for head unit cpu, center is the secondary battery box (stainless, but am going to polish it up and do something nice with it) to the upper right is the 2.4 farad capacitor and lower right is the Kenwood Music Keg
#9 and #10 are just close ups of Land R of trunk
