I have a f150 with a ext cab.....what size subs will fit under my backseat....im thinkin i want 8in subs...but i dont know what fits....

 

Are you wanting to not lift the seat? Without a seat lift, options are very limited...you can fit some nicer 8's under there though. Lift that seat up 2-3 inches and you options increase quite a bit, still not a whole lot to choose from though.

 

There are pre-made boxes for both 8s and 10s. Lift the seat and get creative and you can fit nice 12s but its a bit more of a pain.