Going to try and keep this as brief and informative as possible so that when someone prints it out for a manual to use while installing in their garage they only have a few pages to flip through. This is the Low Beam install. The Fogs and Highs mimic the same process. I'll add bits and changes I noticed between my old 2008 and the new 2012. Feel free to add pointers that you may have as well.
New Guinea Pig....
http://i600.photobucket.com/albums/tt85/ArmaDen/IMG_20111028_175041.jpg
This is what you are starting with: Ballasts x2, Bulbs x2 and Connector wiring.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05905.jpg
This install write up is performed on the front passenger side. Identical for the Driver's side.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05906.jpg
Turn the wheel inward depending on which side you are working on.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05907.jpg
Looking into the wheel well you will have 4 screws you need to remove. 3 on the lip and one inward in front of the tire. These are all Phillips head screws. Locations are noted with Orange tape. Consistent from 2008 to 2012. Should be similar for prior years as well.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05908.jpg
Once the screws are out keep them in a safe place as you don't want to loose them and have a loose fender liner. Push forward in the middle of the lower portion of the liner to be able to pull the outer liner lip away from the fender and towards the tire to open it up the area. Once inside of the liner you will be able to see the rear of the headlight housing and the rear of the Fog light housing(if equipped). You can see those marked by the Orange tape. Lower is the Fog and the upper is the Low beam.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05910.jpg
Close up of the Fog.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05911.jpg
Looking upward you will see the rear of the headlight housing. Low beam is marked by Orange tape on the right and High beam to the left.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05913.jpg
First remove the stock Low beam filament bulb. Twist the base counter clockwise 1/4 turn to unlock.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05914.jpg
You can unplug the bulb at this time from the truck cable or install the HID bulb. Your call. To install the HID bulb slide it into the Low beam hole seen where the stock bulb was removed. Tip: The ceramic coated wire that runs along the outside of the glass HID bulb should be around the 4 O'clock position when inserting so the locking notches line up properly. Turn the bulb 1/4 turn clockwise to lock in place.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05915.jpg
Now connect the two wires from the bulb to the corresponding wires from the Ballast. They are mating plugs which means you cannot connect them in the wrong fashion. Plug in the plug that used to plug into the stock bulb that you just removed and plug that into the other cable(supply) that lead into the Ballast. Should look like this once connected.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05918.jpg
Test Fire. (turn the lights on) Should look like this if the polarity is connected properly. Tip: If the bulb does not fire then rotate the rotate the wire that plugs into the ballast from the truck 180 degrees to reverse the polarity and it will work.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05920.jpg
Now mount the ballast. For spacial reference the upper Orange tape is the rear of the Low beam and the lower Orange tape is a supporting bracket for the bumper cover that has a great flat spot on it to mount the Ballast via double sided tape or Zip ties.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05917.jpg
There is also another location for the double sided tape method on the Crash bar structure. Pic is a little dark but you get the idea. The Fog light housing is at the bottom right of the photo for location reference.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05922.jpg
Rinse and repeat for the Fogs. Tip: For earlier models(04-06) be sure to read up on the potential issues with the HID and Fog light lenses.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05926.jpg
Now one of the trickier parts is getting the liner to line back up correctly. The 2008 you had to slip the grommets by each of the screw holes between the metal fender tab and a plastic backing piece. The 2012 has a circular piece to help this process. Once you start putting it back together you will see the parts I am referencing. Just make sure you slide the liner between the correct layers for secure re-install. Photos for reference.
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05928.jpg
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05930.jpg
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05931.jpg
Lastly you should be left over with 4 items....
http://i600.photobucket.com/albums/tt85/ArmaDen/DSC05932.jpg
To best serve the CA members it should be noted that no matter which aftermarket HID conversion kit you happen to buy....this process will apply. Should be noted that we sell them for $60 shipped per kit. Shameless plug I know.
This also applies to the Nissan Titan (all years) setup as well as I know many of you have an Armada for the Wife/Girlfriend/Mistress and a Titan for yourself.
Questions are always welcome. LED package write up is forthcoming.