Here is what I did to charge my nexus 7 which needed a much more powerful charger than the USB in the car.
Full project is here:
http://mazda3revolution.com/forums/2014-mazda-3-skyactiv-how-guides/63073-nexus-7-project.html
Install USB charger
What you will need to buy:
Fuse Tap ($5)
15A Fuse ($1)
DC Converter 12V to 5V 3A Car USB Charger ($13)
a. Open fuse panel by your left foot on driver’s side.
b. Remove bottom fuse (15A)
c. Remove the black bolt under fuse box with 12mm socket wrench
i. This will be a tight fit, but it comes off pretty easy. If you are having trouble you might be turning it the wrong way…
d. Take 15A fuse you removed and the extra 15a fuse you purchased and insert them into the fuse tap.
e. Using a wire stripper strip the thick black coating off of about 8 inches of the usb charger exposing the positive (red) and negative (black) wires.
f. Strip the positive wire about an inch exposing the copper.
g. Stip the negative wire about three inches exposing the copper.
h. Insert the positive wire in the blue crimp on the fuse tap and crimp down.
i. Take the negative wire and wrap it around the base of the black bolt you removed.
j. Put the black bolt back in under the fuse box making sure to keep the wire attached.
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k. Insert the fuse tap into the bottom fuse port where you removed the 15A fuse.
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l. Run the USB charger up the pillar above the fuse box. There is room for the small box to fit through.
m. Cut one of the 3M stickers in half lengthwise (hotdog) and attach it to the USB charger.
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n. Remove the film from the other side of the 3M tape and mount it under the dash.
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o. Plug in USB micro cord included with your Nexus and wrap once around steering wheel column, then bring it to the mounted tablet.
p. Use zip ties to secure any loose hanging cables.