So I found this piece of plastic trim in the back of my 99 near the jump seats, but I cannot figure out where it goes. Is this even part of the truck, or is it just left over from some kids high chair?

 

Yes, that is part of the truck. It goes between the jump seat and side plastic panel. It attaches to the hinges/bracket of the jump seat. I think it goes on the passenger one the drivers one doesn't have one.

 

^^^^Correct.

If you sit in the passenger jump seat - it goes to the right of you. It hooks onto the hinge/bracket just like buick215 said.

Make sure to remove it before folding the seat back up!!

Took me forever to figure out where that thing went.

 

I don't have to remove mine. I move the jump seat in or out without removing it.

 

For real? I never tried it.

 

Go and try it when you have a chance. It should work unless yours is different, which I doubt it.

 

Awesome! Thanks a bunch!