Well, I have been slacking lately and mostly staying inside, so I failed to post a nice setup that I figured out to run USB to the backseat. My 9 year old daughter is the only one really sitting in the backseat, and like most kids now, she enjoys playing/watching videos on her phone/tablet. So, in order to keep her gear charging back there, and charge my wife and I's phones up front, along with the dashcam power... this is what I did.
I started with this, plugged into the front 12v source.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I ran the cord from the "plug" part up into the glovebox. I then plugged in my dashcam, wife's phone charger, and my phone charger to the outlets on the adapter/splitter. I also plugged in this extension.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I then ran that extension out through the side of the glovebox area, and underneath all the lower trim, all the way to the back seat. I popped the extension through the "fold" in the rear seat and attached my daughters corresponding charging cable to it. This allows her to have a very useful charger, with out seeing any wiring, but also not modifying the car (I originally was going to add in a mounted usb charging port somewhere back there). This extension was the PERFECT length!
Some of these Amazon purchases I used Myvipon.com coupons with, which brought my total into the $20 range for the whole setup, including my daughter's charging cable.
Hopefully this helps someone who would also like rear USB charging, while not altering the vehicle.
Took some crappy cellphone pics-
What you can see upfront:
What's hidden inside glovebox:
Where the extender comes out in the backseat and charger cord is plugged into: