Ben Hur

Mine was not the seat itself or the bolts holding the seat brackets in place. Mine was a little bit of play at the top of the rear shock absorbers. The nut for the shocks is accessible from the exterior above the back tires. I don't recall exactly but i think there is a tab strap two of them at the seat where the upper and bench meet--- where you would lose your coin change. Pull upwards and towards the front of the car. Rotate up and it will unclip from front pins.

What cap? No cap in the photo.

If it is a rattle under the back seat while driving you absolutely nuts trying to find it, try the shock. You can see the top of the shock stem just over the height of the back tire. Take off the tire if need be but you might be able to reach in and remove the plastic cap that conceals/protects the nut on the top of the shock. Use a 6 or 8 mm wrench (dont remember) to hold the top of the shock from rotating. Turn the nut with another open end wrench probably no more than 180 degrees or maybe one turn and tighten that godforsaken loose nut. There is probably 1/64 inch or less of play. Bang bang rattle rattle and it feels and sounds it is inside the car. I tightened mine about 3 months ago. Gone. No issues now. Horrible rattling, gone. It rattled the seat right out of its clip mount more than once. I was looking for a big loose bolt and it was the littel stem shock nut slightly loose. 5 minute job if the tire is off. Seems to be a common problem. 2016 Edge AWD 2 liter Eco. 65000km at the time.

There is a good chance the horrible rattling noise is the shock absorber. The top of the shaft is a little loose. You can see the top of the shock absorber shaft behind and just above the rear tire. I did not need to jack the car or remove the tire but I did because I wanted to see what the heck was going and it gave more room. The 15 mm nut on the threaded top of the shaft needed to be tightened. First I removed the plastic protective cover over the 15mm nut----it popped off no problem. I was able to reach the top of the 15mm nut with a box end wrench. I held the stem in place from rotating with a 6 mm wrench (or pliers). Mine was loose about a half a turn. Fixed, no more noise. I did not remove the shock or remove the mounting bracket. It is was really hard to believe that little bit of play caused that rattle but, it is gone now. 72,000km on the car. It has been rattling for a long time. I found a video on youtub that was for a Fusion----same kind of complaint, same set up on the Fusion. I thought my rear passenger seat was loose or the bolts had fallen out but no, it was that loose shaft.