The converter goes directly behind the head unit. How you connect the amp is another issue. I personally ran new wire straight from the amp to the speakers, eliminating all of the stock wiring. But you could run wire back to the stock wiring harness behind the head unit. That'd be pretty easy to do, but your sending your signal from the front to the back for the amp, then to the front only to be sent to the rear speakers. Seems like there'd be alot of power loss along the way. The stock speaker wire is only what, 18-22 gauge, so replacing it with even some moster cable 16 gauge would be ahuge imporvement.
The rear speakers are by far easier to deal with than the fronts btw. Especially when adding new wire, as the front door gronets still have part of my hand attached.
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My Xterra Pics I wish I would have a real tragic love affair and get so bummed out that I'd just quit my job and become a bum for a few years, because I was thinking about doing that anyway.