Wiring powered sub into 2016 CX-5 GT with Bose package

[S1DC]

So I've been replacing my factory speakers and so far it's been relatively easy. Got the front door woofers and rear door speakers replaced, and the dash tweeters and center channel are done. I'm doing a modification to the rear pillars to replace those speakers, and all I will have left is the powered sub I for under the seat.

My question is, what is the best way to get the high level speaker signal from the rear doors to the sub? I'm confident I can get the power hooked up, but I've been searching everywhere and looking at wiring diagrams left and right to get a fix on the best place to tap into those speaker wires and can't get a solid confirmation as to where is best. Currently I am considering tapping into the wires behind the speaker in the door, and running the new wires back through the door boot to the B pillar. Apparently I'll have to pass through a harness molex by drilling out some unused spots from what I've found so far online, but no actual photo or video examples of doing this. I would rather just tap into the wires in the B pillar but I'm not sure if they continue past the harness connection in the same color and configuration as before it. Any suggestions as to what to do would be great.

-edit

I am opting not to use the pillar speakers for the tap in because I assume they're crossed over to be mid range

 

[RedBaron]

I believe that people usually tap into the wires at the amp, under the passenger seat.

 

[S1DC]

I believe that people usually tap into the wires at the amp, under the passenger seat.

I couldn't find a wiring diagram for it that I was confident I could identify on the actual harnesses. Not enough to start cutting into wires, anyways. I need to see an example of how someone did it with photos or an excellent description if I'm going to go that route. That's what I initially was going to do but someone else I talked to about it said tracing the wires out of a factory amp can be tough.

Willing to do it, just need more confidence I'm doing it right. With the rear door speakers, I know I have the right wires without question and it's just a matter of running the splice to the sub.