Separating hatch and rear door speaker wiring without cutting?

[Kenamond]

I have a 2018 Limited with premium JBL and will be replacing the entire sound system. This includes installing an 8 channel amp. This amp will drive tweeters (dash), front doors, rear doors and hatch speakers all on different channels. Because each speaker has its own channel, I need to somehow connect the amp speaker output for each channel to each speaker, but I would rather avoid cutting the factory wiring. The dash and front door speakers are in parallel, but I can separate them at the 4-wire connector to the dash speaker; two of those wires are for the dash speaker and the other two continue to the front door speaker, so easy peasy.
My problem is that I don't know what to do to separate the rear door and hatch speakers which are run in series. I need a place to tap in that isn't inside the door or the hatch if possible (otherwise, I have to run new wire through the door boots or cut something).
Is there a connector somewhere in the main part of the truck (not in the door, not in the hatch) that I can tap into to get at the hatch and rear door speaker wires separately? Although there are a million useful threads for audio install/modification, I haven't found anything about this.
Thanks!
Mack

 

[Kenamond]

So after taking the winter off, I sound deadened everything and while the headliner was (mostly) down, I was able to find a baby blue connector on the driver's side between the roof and the headliner that contains the liftgate speaker wires. Rather than cut those factory wires (which are easily accessible through the side panels for accessing the tail lights), I'd rather depin the factory speaker wires from the connector and substitute my new wires, crimping new terminals on the wires.
Anyone know which pin type/size I need for that connector? I'm not sure if they're 18ga or 20ga or something else.