I know this is old, but it might help some people:
If you can afford to purchase a new upper case assembly, just do it. Buy one with the trackpad, keyboard, battery, etc. As much fully assembled as possible. This will save you so much time and hassle and is pretty doable on your own.
On the other side, if you are like me and couldn't afford to basically almost buy a new computer, you can buy the keyboard/led sheet online on Amazon or here. Make sure to also buy the microscopic screws you will need as well.
I would also buy a replacement battery kit from ifixit. Buy it to not only assist you in removing the battery (which you will need to do to get access to the keyboard), but you will have a backup battery if you have screwed up the original one (which you most likely will do. Any sort of bends or anything basically ruins it.).
After you use the guide to basically tear down the computer into a million pieces, you will get to the keyboard that is riveted into the upper case. Yes, like 75-100 little rivets are attached to the case. Apple hates you.
There is not really any great way to pull the keyboard out, you basically have to rip it out using your hands (with a glove) and or some pliers.
The ONE thing I would recommend is do your best to rip out the rivets ENTIRELY. The rivets that end up busted and are still stuck in the holes will have to be removed (to the best of your ability). This is where I was about to lose my cool.
I suggest using nail clippers to take the remaining posts out. Clamp the post and wiggle it slowly out. I don't know how else to describe it. Some of them will still be stuck.
Any that you can't get out with the clippers, angle a flat head against the rivets and give it a gentle tap or two. Hopefully some of them come out and the rest will end up just welded into the holes forever.
Do all of this with extreme caution and know that if you mess up too bad you will probably have to just find a new aluminum case or the full assembly.
I would say roughly ten of the rivets on mine did not come out all the way. Luckily there are lots of small holes and tons of small screws to adhere the keyboard to the case. Some of your keys might end up having a bit more play in them than others... But that's what you get for being cheap. I am a student concentrating in programming and I use my Mac all day all the time and the keyboard has shown no signs of stopping now. Using an external keyboard all the time seriously sucked.
Bottom line: Don't be cheap, buy the uppercase assembly or find one off eBay. If you don't, you will have no one but yourself to blame for the frustration... Well yourself and Apple for literally making it almost impossible for you to fix your computer yourself.
Edit: Also, use an antistatic mat/wristband/whatever you need to make sure you aren't going to ruin your 1200 computer. Buy some static bags for the logic board or other parts. Seriously you will be working on those rivets for a while and best not to risk something stupid like frying your board.
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