2017 Chevy Equinox i4 stereo wires

I'm trying to install a subwoofer by tapping in to the rear factory speaker wires either before or after the amp. I found what I thought was the only amp. If I unplug the top(power) or middle connector, the front speakers stop working, but the rears still work. If I unplug all 3 connectors, the rear speakers still work. Where is the rear speaker wires so I can tap in to them for a new sub? Ideally pre-amp. I've started taking the rear door panels off, and have a low-level line converter, but would prefer to go pre-amp if possible. I've searched around and there's a lot of info for older equinox's and for the 6cyl version which is different. Images of the amp and wires https://imgur.com/a/6nBG6