depends on vehicle, but usually i advice against it...
reason being, one, mostlikely you have to do some custom fabrication to get anough of an ecnlosure space, two, you wil have to use a shallow sub, and three, it fires rightinto the seat cushion, and on most cars, the fitment is not that great...
so basically,y ou spend a lot of effort in creating something htat doesnt get very loud and dont sound that great...
the proper way to do in floor subs is to cut a whole inthe floor, weld in an enclosure, and then put a sub flush into the floor

but you can try bass shakers, from aura, little virbrating gadgets that simulates the rumble of a bass note, surprisingly effective...especially if you have some kind of small sub somewhere that initiates the acoustic part of the bass frequency.
b