Very detailed and helpful write up!
I'll add a few hints from my own 2007 Altima :
1. The OP stated it, and I'll re-emphasize: The very first step to this install is removing the negative battery cable. It is important.
2. When running your amp wire through the firewall, also pull an additional power and ground wire to connect your new deck directly to the battery. Almost all white noise in a stereo system is related to the deck and amp grounding points. It's a simple step, but it can go a long way in keeping engine noise at bay (especially if you are running a 4 channel amp for your mids and highs).
3. When grounding your amplifier, be sure to scrape all of the paint off of the grounding point. Your best bets are items that bolt to the chassis of the car. I used the rear seat belt bolts, and simply used a flathead screwdriver and wire brush to remove the paint. To get to the seat belts, you just need to remove the butt portion of the rear seat. Its tremendously easy. See the two pull rings sticking out on the front of the seat pad? Pull on the ring and pull up on each side. Now remove the seat.
4. During installation remove the faceplate from your stereo if possible. Frequently put it back on to ensure clearance, but with all the metal tools floating about, you could be asking for trouble.
5. If you don't have automatic climate controls, purchase the factory 2-din dash kit. It gives your new deck a completely factory look.
From member Sethman408:
Order it here: Nissan Parts 2 U
It is part #15.
Then order this deck pocket: Universal DIN Pocket at Crutchfield.com
The OP did a great job on his install, and it turned out great, but the factory panel is like the tire shine on a freshly waxed car. It just sets it off.
For installation using the factory dash kit, you'll reinstall the climate controls and metal piece from step 6 (held on with torx screws) onto the new dash kit. You'll then screw the deck and pocket directly to the original metal piece.
From a security standpoint, the factory dash kit is far superior to the other aftermarket options that utilize the metal cage that comes with the new stereo. It takes a thief about 10 seconds to rip a deck out of the cage using some flat head screwdrivers; say adios to your stereo. A thief will steal your deck with or without a faceplate ( I have two faceplates without stereos as proof ). With the factory kit, a thief will have to remove your AC vents and lower pocket panel to expose the 4 screws shown in steps 2 and 3 of OP. This takes additional time, and that is a deterent to thieves.
BONUS Security tip: When reinstalling the factory dash kit once you've bolted on the Climate controls and new deck, go to the store and get new screws for the four screws shown in steps 2 and 3 of OP. I use screws with square inserts (commonly used in home electrical). Once I've installed the screws, I use a drill bit or allen head to strip out the head. To remove the screw you have to use pliers and it takes a while, but a thief will not bother with it.
BONUS Security tip 2x: Use these same screws for installing your amplifier. I mounted mine to the back of the passenger seat. Don't forget to strip them out.
BONUS Security tip 3x: If you place your subs against the back passenger wall like the OP did, you can actually chain them to the rear speaker deck. You don't need a monster sized chain, this is simply a deterrent. Remove the subs from the box and install an eyelet that bolts from the inside of the box. Now run a chain through the eyelet and through a few holes on the rear speaker deck. Be sure to wrap the chain in electrical tape / duct tape to eliminate rattling. Use a combination padlock to lock the chain; put the combination in your cell phone. If you use a keyed padlock, you risk losing the key; not good.
A thief will steal your stuff if they really want it. You can't stop them. These steps will certainly help to deter them though.
6. Depending on how detailed your install has been (deck, noise deadening, door speakers, remove seats, remove door panels), you might find your auto up function does not work on the driver window, and the air bag light stays on in the dashboard. Easy fixes:
Window: Roll both front windows down halfway. Now roll them up and hold the up buttons for an additional 5-10 seconds.
Air Bag Light: Turn your car off. With your foot off of the brake, turn the car to the "on" position. In the instrument cluster the airbag light will blink, as soon as it stops blinking (but before it turns on solid a moment later) turn the car off. Wait a few seconds and repeat this step 3 times.