I believe it's a fairly common issue, but granted that I've only been around these Cayennes for a short while.
1-Unable to release latch to tilt seat-back forward, failure to unlatch.
2-Seat latch fails to secure seat-back in the upright position, failure to latch
For me, I was running into failure mode #2. When pushing the seat-back up it would not latch no matter how hard I tried. I sourced a new-used replacement from ebay.
The parts between 955 and 957 are interchangeable. Unsure about 958.
-torx bit/driver/key, T20 I believe, or T25
-triple square bit, I forget the size but can update later
-wrench that fits triple square bit, I used a ratchet with a 13mm socket
Step 1: Remove two torx screws, T20 I believe.
The first screw is partially hidden by the latch lever, pivot the lever and remove the screw.
The second screw is installed from the side, in the opening of the plastic latch cover. The plastic on my cover had cracked around where the screw was inserted, so it is shown here after removal.
The entire plastic cover assembly is now free and is removed by sliding up along the same axis as the first screw. If you are experiencing failure mode 1, at this point you should inspect your plastic cover assembly. It is possible that three tabs on the lever can break off as shown here. If this is the case, you simply need to replace that part and not the entire latch.
The latch assembly is held on by two triple-square screws which are visible once the plastic cover is removed. Remove these screws and the latch will be come loose. Then there is a plastic clip that holds the upholstery to an edge of the latch, shown front and center in this photo, that simply pulls off.
The latch, removed from the seat back.
Here is the top of the latch, this cup with the three slots corresponds to the latch handle that should have three tabs.
Another option for failure mode #1 is that the latch itself had a broken component, there is a 2-prong fork that should cover the front and back side of this post, as you can see the rear prong is broken off. That post is connected to the cup, which the lever is assembled to. So when you pull back on the lever to release the seat back, it turns the cup, and rotates the post against the fork, which releases the latch.
In my circumstance it seems as if it was a 2-part problem. My fork was broken, but when I purchased the truck my problem was failure mode #2, not being able to latch it. My theory is that the broken part of the fork lodged itself into the latch and prevented it from securing properly.
Next, reassemble with new or used parts that are not broken. It's pretty self explanatory here with the exception of lining up the plastic cover/lever assembly. You need to pull the lever as if you were disengaging the latch, and then feed the plastic cover assembly down ensuring that the round 3-prong section aligns with the 3-slot cup.
I have heard that there is some adjustment you can do with the post that this latch connects to, the post mounted into the chassis itself. Adjustment here, if it does exist, would move it slightly so that the latch would travel further over the post and enable it to latch. Upon testing my new latch worked fine so I didn't explore this option.
Finally, you want to keep that latch working well. From now on when you want to release the seat-back, first push the seat-back back and then while holding it pull the lever to release. This puts far less strain on the release mechanism and will prolong it's life.