ddont knw oyour budget but lets assume you are doing okay but dont want to break the bank on speakers
1st of all dont go through the hassle of under the front seat been there done that (four boxes) and came to find out that there is no way to eliminate the rattling inder the seat since there is no foam or fabric under the seat just open metal. and sound waves against metal are a big no no
so anyway but if you must you have a mouting depth of no ore that 5"
so that leaves you with
1) Resonant engineering 8"
2) elemental designs 9kv.2
3) image dynamics id8v3
4) kicker l78
or even a adire audio extremis 6" or RE XXX 6.5 mid sealed (these two may work under the front seat very well)
for a little fun check out the excursion video on the extremis ( the little 6")
its 10mb file
http://www.adireaudio.com/Files/Extremis6.AVI
components
that all depends on your taste
sets I like
1) DLS Iridiums 6.2 (I have the 8.2 but that takes too much work)
2) focal k2p ( but the tweeter are really bright like the mbquarts)
3) morel elates--bst Ive heard but they a really expensive like 900 bucks
4) alpine spx-177r (low end in comaprsion to these but nonetheless the tweeters/crossovers are awesome-mids just okay
5) kicker ss65.2 very good set if you can catch them off ebay for around $300
spend you big money on your front speakers,and put a lesser set in the rear, once you out your sub in, the rears dont add that much to the sound stage anyway but others may think differently
I have heard all of these and they all are very nice, there are tons of other sets out there, others can chime in
If you want to keep the space I recommend taking one side of your rear cab, and put two smalll 8" subs, or 1 10" ( 11kv.2 or a id10v3.2)
for SQ , the both need small power,and small boxes. give it 0.8 cu ft and you are good
amps the best 5ch I have used
is the usamps 5600x 75x4, 200x1 into 40hm, but the sub channel can run at 2 ohm for 350w, thats enough for the two 8's or one 10
these are hard to find, but ID highly recommend to get one soon as usamps just got sold so the amps may not be as good as they once were
knuconceptz.com makes the best cost vs perfomance wire kits on the market. their power wire is excellent mroe strands (more flexible/more current) than anything on the market at half the price, rcas are nice, distro's etc..
there are many car stereo folks who have done it
do a search for stereo , aftermarket sub box to find many good ideas
good luck