I bought my Jeep used, and upon cleaning the upholstery for the first time, discovered that the passenger seat release strap was missing. Easy fix.
My buddy and I pulled the seat cover up a bit to access the bracket inside of the seat where the strap attaches. You will see a silver cable that the original strap attached to, with an oblong fitting on its end.
We used military parachute cord, 550 cord, for the fix. We used desert tan, and it matched my tan interior perfectly. Make a loop of the parachute cord to the length you want it, then you take two cable ties and tie it off to the release cable. Play with the mechanism, and you see how it works. It helps to have a pair of long nosed pliers to grab the cable and hold it while you cable tie the loop in place. Pass the free end of the loop through the bracket and out of the hole in the seat cover.
MAKE CERTAIN THAT YOU TEST AND ENSURE THE MECHANISM WORKS PROPERLY TO LOCK AND UNLOCK YOUR SEAT AS DESIGNED FROM THE FACTORY!! You do not want that seat to unlock on a heavy stop or during an accident!!