Lumbar adjuster lever (for driver seat only I believe)
You'll find a hole in the right side bolster of your driver seat approximately at the height of the armrest.
The round cap comes off so you can screw it back in, though I suspect it is damaged from breaking off its post. If so, then just use some pliers to turn the post to the desired position and throw that lever in your junk drawer...
Funny thing is I had my GT Spyder for five years without even knowing it had lumbar adjuster. I found out when I swapped to leather seats, and was pleased to discover the lumbar is much more comfortable on my 65 yo back.
Also took me that long to discover the audio controls on the back of the steering wheel. LOL