I found the most bass responce when my subs fire stright back at the back doors.
Not much from them firing towards me.
For a long time I drove around with my subs firing up. (as you see in my sig.)
I got a real good responce from them but...
when I turned them babies away from me (towards the back doors) ,
Oh my, it was amazing!
My friends swore I had a new amp.
Set-up is everything!!!
With one sub, it'll be different... But, I think you'll find the above will steer you in the right direction.
Slap together a cheap mdf box, and play with different placements.
Then go build that fiberglass box formed off the wheel hump.
You'll always know it's in the best place, for that sub, for that suv.
Just my 2 cents worth,
"B"