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There's a youtube video of a guy making his own harness to bypass the Bose amp.
Don't know if this helps but I added a dual 12's in a fairly large box in far rear of my Denali prior to aftermarket h unit. I bought the "audio control lc2i" which is a line out converter which I tapped into the far rear d side pillar speaker for signal to subs and turns on amp when key on. So I could retain the use of all Bose speakers. If I had ran off h unit only four speakers would work. I put my amp and l.o.c in the area you mentioned. Honestly it's not very hard at all to run the power from battery to the rear in these vehicles since it's pretty much open after you get to that rear panel. I can take a few pics if you like so you can get an idea. Audio control makes a lc1i converter now too. Unfortunately if u want to add a aftermarket sub you will need to run power wire to the amp from battery. Lmk.
I installed 8” touchscreen, backup cam, and have had 2 industry Titan 15’s in the back. No 3rd row. I did however build a box for twin 12’s that fits between the 3rd row and hatch(Non XL), that sounded pretty good. As of now, picked up a third car, so the Tahoe is now my Hoe. 
Yes I have 05 Yukon and added a sub and amp. I did not connect it to Bose factory. I kept stock speakers since they sound fine even with 2 12" subs. I use the "audio control lc1i line out converter and tapped off far rear cargo pillar speaker for signal to amp turn on for subs. Great little device. Check it out. There are cheaper less expensive loc's but this gives you much more control and ease of function.
