What you are experiencing is a typical failure of the kick down cable.

You may be able to extend its life, but you are looking at a replacement to fix the problem. Take it loose (13mm nut)and hold it up so gravity works for you. Add a little PB Blaster to the cable and allow it to soak down the cable. Then add (a drop or two at a time) ATF and allow it to run down the cable into the sheath.

Take a pair of pliers and grip the cable about 1/4" ahead of the sheath (get that red cover out of the way) and force the cable back into the sheath. Repeat the pushing of the cable until it is back enough to then be pulled out again. Keep using lube and cable movement until it (hopefully) moves freely.

If you can not get it to move freely just push it back into the sheath and leave it disconnected. In this position the car will want to shift long before it should but this can be overridden by shifting manually through the gears; much better than having it overrev and not shift because the cable is out too far.

Randy