Now, reconnecting this one will be very tricky. Here's what I did after several hours of frustration:

Before flipping the board back down, make sure you left the flat cable locking piece completely open

Flip the board down and reconnect all other cables, except the "flat bastard" one

With a spudger or using your hands, lift the main board slightly up, enough for the flat cable to slid into place

Once it is there, release the board down. Forget about locking the "flat bastard"'s connector locking piece, you won't be able to do it!

I found it easier to put the "flat bastard" in place using twizzers. Whatever you may use, just be carefull to not damage the cable or the connector!

Make sure it is in place, then secure it using a piece of kapton tape or any other tape you might have on hand. This will prevent the cable from coming out of its socket when you move your computer.

Resist to the temptation to pull it back, it would only make you need to do the job twice.