2000 Crew Cab SE Subs?

I took the backseats out and am in the middle of building a box for a 15".
The backseat was too small for people anyway.

They make boxes to fit 2 8" or 2 10" shallow mounts behind the backseats or you can make your own.
I personally hate shallow mounts, most of the one's I've heard don't sound too great and have as much bass as a nice comp stage.

The new Alpine Type R shallow mounts are really nice.

If you're wanting stright SQ then Tang Band and Elemental designs makes some nice 6.5" subs that would easily go behind your back seats.