I had the same problem - while replacing the wifi antenna I must have knocked the white tab on connector 3 - it was at a funny angle and when I tried to press it down to lock down the #3 ribbon cable (screen phone earphone and proximity sensor) it pinged off.
[Lots of four letter words in capital letters]
I spent ages looking for it and found it - it is approximately 0.5mm wide, 0.1mm thick and 5mm long.
The tab has 12 holes on it to fit on the 12 pins of the connector. When the tab is at 90 degrees the pins are level - when you push it down flat, it levers the pins up (on the other side of the connector the pins are levered down onto the ribbon cable making contacts and physically securing the ribbon).
This thing is absolutely tiny. Saw a youtube video of a repairing of these and he had a microscope. That is really the ideal kit. I went round the house looking for a magnifying glass - we didn't have one. I then tried my wife's pocket camera on macro mode - it just wasn't able to focus close enough to help. Found some binoculars but they had a min. focus distance of like 15 feet.
Eventually used my dslr and jury rigged a macro extension tube with a cardboard tube. This gave be something like 5-10 times lifesize and I could actually see the pins. My dslr is old, but a modern one you could have used "live view" and had real time images. I just took some photos just to see what I was dealing with at high magnification.
I then used another telephoto lens reversed as a loupe (not connected to a camera - just the lens with both caps off as a sort of super-magnifying glass). Bear in mind this is just because this is the only lens I had available - microscopes/magnifying glasses would have been preferable. I didn't have any tripods or stands so I just proped things on a tray.
With all of these optics, the depth of field is miniscule - I gave up trying to change focus and just used fine changes in distance to focus on the bits I was interested in.
Light is very important - I was using a 3 cell maglite with an LED conversion. This is super directional and I had a nightmare trying to focus / position the lens while also trying to position the "spot" of the torch. I had a brainwave and unscrewed the refector end of the torch which then turned it into a very bright unidirectional lamp. Again - this is what I had to hand, a good angle-poise lamp would have been good.
I then tried putting the tab back on the pins using a pair of tweezers. This is pretty impossible with normal tweezers (I used the pair from a Swiss Army knife). Because one edge of the tab is chamfered (so you can get something under it to lift it in normal operation), it has a tendancy to "ping" off like a grape pip squeezed between your fingers. Finding something so small is a nightmare. I did this for about an hour.
I finally just placed it in front of the pins and just used a jewellers screwdriver to nudge it into position and slipped it onto the pins.
Putting it back together is actually not that hard if you can see what you are doing. An industrial inspection microscope has built in lighting and would have been perfect for the job.
Unfortunately I couldn't take photos while I was actually getting it back on (holding lens / phone/ light / screwdriver took all the digits I possess) but it looks just the same as it did before and now snaps back and holds the ribbon securely. Phone earpiece and proximity sensor both work fine again.
Yay!! Replaced wifi antenna have wifi working without standing next to router and recovered from breaking the motherboard in the process!