01 V70 new sub and amp wiring

[CIK7]

I am making plans to put in a new pair of JBL GX602 speakers in the front doors, as well as a 6" Pyle sub in the center dash. The tweeters are weak, front speakers don't work, rear work decent enough. First: Is it true that the dash speaker goes directly to the head unit and not the amp? That's what I read somewhere I think. Would this 6" sub be an ideal solution in the center dash? I don't want a box to take up cargo space because it's all important for me. I mainly use it as the bed area for my dog and for big stuff like giant couches. Second, where would I find the harness for the center dash speaker to hook into the head unit? In the picture I posted, you might see that there is a wire on one underside of that opening. What does that go to? My car came with the 603 or 613 head unit and was switched to the 813 head unit. If y'all know what the harness is called and can find it on eBay, please let me know. Thank you.