I placed BOTH my subwoofer monoblock amp and my custom subwoofer box (.47 cu. ft. sealed) for a Pioneer Premier 10" Shallow Mount behind the passenger seats. Just say to heck with it and remove that rear kick panel, the spring-loaded seat back flap-thingy, and also that black plastic divider on the floor the spring-loaded flaps screw into. Doing this gives you a the added room you'll need to construct the shallow mount enclosure... and if you're putting in a sub at all... you totally going to need to put sound deadening material all along that back area and your doors. That back area has some horrible buzzes and rattles without the sound deadener and without shoving foam behind the sub enclosure and various other plastic parts of the kick-panels left behind on the sides after removing that main one.
The amp for my doors I have placed underneath the back passenger seat and I can easily fold-down the seats as per normal... minus the spring-loaded flap thing to take-up the gap from back of seat - to rear of cab.
When you put an amp under that backseat... don't be dumb like I was and cut into that vinyl flooring material in order to mount directly to metal. That totally not necessary. You can just screw into that vinyl material itself and it'll hold it in place just fine.
About the only cutting to that vinyl flooring you MAY wish to do is cut a slit behind where the door amp will sit in order to more cleanly and invisibly route the wires/cables to the back of the door amp from the side cable trench.
In all honesty, I can't begin to imagine how buzzy/rattley it would sound if I did an under-seat install of the sub and left that rear kick panel intact.
For reference purposes, I'm pushing 600W RMS to the sub @2 Ohms and 50W RMS x 4 @ 4 Ohms to the aftermarket door speakers.
P.S. don't even bother adding amps to your system until you get an after-market headunit with 3 pairs of low-level outputs (Fr/Rr/Sub). The difference in sound quality is totally noticeable from when I was using line-out converters tapped into the existing "hot" wires feeding the rear speakers. Sound TONS better, and the new head unit will give you greater control of the sound as well.
Side Note: My rear 6 x 8's have had the woofers blown during that period where I was piggyback'ing the sub off them, probably because I was cranking the output to that rear and not hearing what is was doing to those door speakers since the sub has soo much greater presence back there. However... when I put in my new Head Unit... it also happens to have a HPF (High Pass Filter) I can turn on which only affects the (Fr/Rr) channels... so I was able to set the HPF to 80 Hz... effectively overcoming that blown 6 x 8's woofer by only sending frequencies to those speakers that are higher than the low-lows which were causing the blown woofer to make itself known. I'm quite happy with this new head unit. A Sony CDX GT630UI. And it was only $124.00 at sonicelectronix.com
