Some one already has installed a Sub under the seat try searching. It was done by tapping into the existing wiring and running power from the footwell power.
Ah excellent - I would prefer to avoid splicing wires though, assuming we are talking about @Eric.S. post: https://www.theineosforum.com/threads/subwoofer-adding-guideline-not-official.12412589/
Considering putting in an amp for the door speakers as well so could get bit messy with all the splicing and needing both L/R channels as well.