can i mount a sub directly in the floor of the trunk? You know how that piece of wood lifts up to reveal the spare. Could i just cut a hole in that and mount a sub in it?

you could but it would sound like shit... also your spare tire is kinda in the way. however if you want to do that idea you need to take your tire out. you can then build a custom fiberglass enclosure in its place. theres a thread below this you can check out

or you could just build a box out of 3/4" mdf that will slide down in the spare tire area and then make the top of the box cover the floor of the trunk. that way you could glue the carpet down and it would look completely stock and that stupid ass carpet wouldnt move around.