I will spare the details but this is to help anyone that will encounter a problem that I did. I have a 2010 C-350 sport . I read a lot of forums and googled a lot to try and find how to push back in my head rests that have been popped out due to a rear end collision. I had no problem with the instructions to get in my passenger side head rest(no one was in this seat when the collision took place). **With the instructions of pushing down on the top of the head rest all the way and then pushing into very hard till you hear it click. Worked great. I did it by myself and was not that hard at all... Now I was beginning to get very frustrated with the driver seat and why I could not get it to click into place using the same technique. *** I finally discovered probably a 1.5 inch silver nail looking piece sitting in the bottom of the head rest. This was found by shaking the seat and hearing something rattle. I saw the piece stuck way down in the bottom of the detached headrest. It is to far down to get with your fingers and to narrow to get a set of pliers. I used a flat head screw driver with a magnetic tip to fish out the piece. You ask what is this and why do I need it??? This metal piece is the way the headrest latches back in.Without it you will not be able to latch it. *sitting in the back seat looking out the front of the vehicle. You will see an empty space almost like a square in the headrest. The piece will be placed in there parallel with the ground. This does require you to slide it in at an angle and bring it back until it snaps into place. Should be able to see this. In my case when the head rest detached during the collision the piece fell into the head rest. I am lucky to have found it and have not seen anything about this exact thing on on web. I wish I would have taken pictures to make this easier to understand. Let me know if you need more help! Hope this helps anyone that is very frustrated😃