man that would have to be a really shallow sub if your bench is like mine. but maybe theres enough room to do it in that triangle-shaped area in the center. I guess it depended on the volumetric needs of the sub's enclosure.
Either that, or maybe consider one of those rear-end-vibrator under-the-seat things, I understand they produce a similar effect without the speaker itself, but I cant be quoted on that.
just saw it in a mag once...
like Rumble-Butt or something.