Just finished installing additional 12V socket. I'm using Garmin GPS on the driver side, so running power cords across the dash was unacceptable.

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Finished Install

12V socket I got from Bass Pro Shop. This one is expensive marine grade socket (around $22) and also very hard to install (with this one you will need to get your hand behind the dash to screw the back plate/ring to keep it in place). I removed part of the dash to install this one.
Next day I stopped at Radio Shack and found another type of 12V sockets, they are around $7 and they just pop in the hole, so all you need is drill a hole, slide wires inside and pop the socket in. If I found them earlier, I would definitely got the one from Radio Shack.

This is the drill bit I used.

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There is ground point, on the driver side (you will be able to see it if you look under the dash). BLACK wire is the GROUND wire for the socket. You will need 10mm wrench.

POSITIVE (+) wire from the socket with 15A fuse. I used heat shrink for nice look.

Here is look of the fuse block (driver side / under dash)
This one is 12V constant power (even when ignition is off), but it can be changed to switched.
This ones will go in the Glove box: 120V socket, USB Charging Port, USB IPod controls for my Pioneer, AUX IN, and additional 12V socket.
Good Luck.
If you have any questions, please feel free to ask.
CUBE