I have a standard cab full size truck but don't want to sacrifice room and comfort by adding a box behind the seat so instead I'm looking for a good sub that works well without an enclosure, does anyone know of any ?
If you want a small box sub that has good sq and decent output go with an image dynamic idq 12 v2 (.55 cubic feet sealed plus .07 for the subs displacement. (.62)
Where would you put it. The reason I ask is because you would have to completely block the front off from the back... In a truck you would pretty much have to build a box to do that... Unless you plan on doing some chopping, but it wont be worth it to chop up your truck for bad bass.
You could also look into shallow mount subs if you really wanna conserve space.
If I were you I would get a nice 8 or 10 in a sealed box and a set of amped component speakers... You could get some nice SQ that way.