I had the exact same issue. Mid 2012 15" Retina MBP.
I pulled the ribbon connector out of the socket and re-seated it. I then lost THREE columns of keys, (Below the 8 and 9 keys now).
I figured since I made it worse, maybe I could make it better.
I flipped up the retention tab on the connector and inspected the pins where it connects to the logic board. It looked like there was a little piece of dust or corrosion or something.. who knows, between a couple pins.
I dipped a toothbrush in some alcohol and lightly scrubbed at the pins, and got the foreign object to come clear. I then blasted air into the connector to dry it out and blow any dust away.
Re-seated the connector, and IT WORKS. So I'm typing this on my fixed MBP keyboard.
Worth a shot. If it's already &&^&@@, might as well try to fix it.