Apparently this is a thing. It said the factory sub is around 2 ohms and can take 100-150W so a 150-250W amp is the right choice.
Copilot recommended these items:
Use the rear full range speaker outputs into this box which converts to a sub signal
https://a.co/d/07u9K13d
Use this for the amp
https://a.co/d/0iUyXqhl
Then it's just a matter of doing some wiring. I could tap into the speakers right at the amp. The battery is also in the trunk. It would be a half day project but conceptually fairly simple.