When I used to install, one of the things I did a lot was build a small, sealed box for some 10's under the rear seats out of fiberglass. It's not hard and only takes about a day. The amp(s) can easily fit under the front seats in most pickups. I think JL still makes some shallow mount subs that'll fit under the seats and don't require much space and are really good quality.
As for the rest, no matter what speakers you put in the doors, be sure to seal and sound deaden the hell out of the doors, both the outer and inner skin, possible the interior panels too. It makes HUGE difference in the sound of the speakers and cuts WAY back on road noise. The only place that creates more road noise than the doors is the roof, so if you can, sound deaden the roof too.
To select the right speakers, take some music you know REALLY well, and is of high recording quality, and listen to as many different speakers as you can. The speakers make the biggest difference in sound and will easily make or break a system.
The RMS power rating for speakers are NOT requirements! Some salespeople will try to tell you that you NEED too send the speakers/subs the rated RMS power, but they are full of poo. Sending a speaker on half it's rated power will only cut its output by 3 decibels. It takes a 10 decibel difference to make something appear half/twice as loud as before. I have a pair of 12's that are rated at 1K watts RMS each and I only give them 200 each, yet in my last truck they got loud enough to be head outside the vehicle over the running 13 liter diesel.
If you aren't installing the gear yourself, then leave that same music in the car that you demo'd the speakers with and make it plainly visible when you drop the car off for the install. The installer will see that and use it to tune with, you may even instruct the installer to tune with them. I cant stress this part enough, you NEED to listen and have the installer tune with the music you listen to and know the most. I have a story to illustrate the point:
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I had a customer bring his SUV in who wasn't very happy with the install that another shop had done because it didn't sound right to him. He asked us to take a look at it and recommend any changes in gear and agreed to pay an hour of labor for this initial look at his system. When I pulled it into the bay I started rummaging through the storage compartments until I found some CD's. I took a look at the system, the install was sound and the gear was very good.
Then I used his CD's, and some similar ones I had, and completely retuned his system from the ground up, including using my O'Scope to set levels. When he came back, I asked him to take a listen and after just a few songs he tuned it off and told me it was the best sounding system he'd ever had in a car.
The next week he dropped of his wife's SUV and spent over $20k on the stereo, one of my alarms installs that COMPLETELY hides in the factory wiring($3K on labor) and some exterior mods.
That is the difference a proper tune can make.
