Bazooka A100 subwoofer install advice needed

[LugNut]

It's awful outside, so I'm working/doing research inside but am still swimmy-headed this July 4th morning ...

I have a Bazooka A100 (6.5 in. powered subwoofer) to install in the Yaris (2007 2DR Liftback), and am investigating the wiring. I'll be putting the woofer on the rear seat floor, on the passenger's side with the speaker pointing out to the side/corner.

I have the rear side panels and door sill off as part of the rear speakers' replacement (done, except for these panels/sill). I'm also removing the rear seats as part of the reversible conversion to a two-seater.

I have a new Scosche line output converter (FA13A) that I picked up yesterday at Walmart, if needed (under $19). After it gets too hot in the garage today I'll do more searching/reading on the site, but if you have done this specific install I'd appreciate your early advice!

I'll be running a positive lead from the battery, probably on the right side under the door sill.

I think I can ground under the center arm rest. If not that, under the right seat mounting bolt -- I'll be doing the seat-tilt mod too.

I'm sticking with the factory head unit, at least for now. Does it have a amp/antenna/accessory (radio on) output? If not, is there an easy place (but hidden) to tap in for ignition-on + power?

How about speaker outputs to connect to? RCA connections on the OEM head unit? I think I read here "no, you must use the line output converter." I have easy access to the rear speakers' terminals right now, so I could solder leads from them back to the line output converter. The A100 has both RCA ("low-level") and speaker wire ("high-level") inputs (as well as an auto-on feature that's not highly recommended in the instructions).

Thanks in advance!