The point of the line out converter is to get low-level RCA inputs for your sub amplifier. Aftermarket headunits already have the RCA output so that's why it's easy to wire directly to the amp.
The LOC INPUT just needs to tap into your L and R speaker outputs from the OEM headunit. You could just tap into the speaker wires at the rear speakers.
Then from the LOC, you just run RCA cables on the sub output into your amplifier that will power the kicker sub. Would look like this without the extras:
Look at diagram #1 from audio control's doc on it, that's exactly what you're trying to do. It's very simple:
https://www.audiocontrol.com/downloa...sales-note.pdf
BTW...if you're still looking for an enclosure, I have a 10" subwoofer enclosure from IntegrityConcepts that is custom fit to the trunk (with a JL Audio 10W0v3-4 speaker in it). I sold the BRZ so don't have a need for it anymore. Looks like this:
