Anything is possible with enough thought put into it...And fiber fill added to the box will mimic the subs being in a larger box (I can explain this in further if you want), even if you cant build the exact box size the subs recommend..

I've seen 2 -15's in a corolla so Im sure you can get 1-18 in an Xterra Just dont expect it to sound good unless you soundproof the heck out of the rear hatch area..