Almost six years later, I buy a 2019 ZL1 1LE with a little over 21k miles and the same nonfunctioning passenger side seat back release problem... here's what I can share about my DIY efforts to fix it. For me, no parts were required; just tools and time and patience.
The torx size is T20. A quality panel tool is handy for helping to get the plastic pieces out of the seat. You'll definitely need a pair of pliers to grasp the cable and reseat the end to the release cable. You do *not* have to remove the seat from the car.
Here are the tools I used:
I didn't start taking pics until halfway thru, so sorry it's not a complete step-by-step. But removing the trim piece to get to the assembly with the actual release handle is the first step. This is where I employed my $40 Snap-On panel tool and worked carefully not to break any of the clips... that part is 50 bucks!
Once you have the trim removed, remove the T20 screws that secure the latch in place. Then there are some clips to be released to get the latch assembly to come out (see last pic below)... you'll also have to stretch out the opening in the upholstery (seat skin) to remove it. It will probably seem impossible at first, but it can come through that opening.
If your nonfunctioning seat back release is like mine was, the cable will be attached to the bottom side of the handle and have come disconnected from the mechanism down inside the seat back. The process for fixing this is to remove the cable end from underneath the handle, clip the bottom end back to the lever arm inside the seat, and then reconnect the top end to the release handle.
This is the bottom of the handle assembly:
Closeup of where the cable end snaps in...
I used my grabber tool to *carefully* extend the bottom end of the cable to the slot where it goes inside the seat back, and then used a fair amount of force to get it to clip in.
If you drop the cable inside the seat back and can't retrieve it, I'm guessing you might be F'ed... to my knowledge, that little cable by itself is not available from GM.
Next, reinstall the release latch assembly. It'll take some stuffin' to get it back in. Once it's passing through the opening in the seat skin, you'll need to make sure it actually clips into the metal structure inside the seat.
Once you have the latch assembly properly seated, you can use a small pair of needle nose pliers to get the top end of the cable re-snapped in to the bottom of the latch handle. The seatback will need to be released (i.e. tilted forward) so you have enough slack to work. Good luck!
Once the top end of the cable is reattached, all that is left to do is to put the torx screws back in and then gently snap the trim piece back into place... the last pic below helps you to know where the clips you need to seat (or release, in step one of this job) are located.
Going forward, I recommend being gentle with that seatback release handle when using it... I only use the handle to release the latch, and then I use my other hand to tilt the seatback forward by applying force to the seat back itself.
