I just completed the leather upgrade for my Screw. After installing the rear seat back in, I realized that I forgot to put the plastic spacer in between the small jump seat and the large seat. Is this part important or can I leave it out.

Are you talking about the c-shaped black plastic piece? If so, that can be snapped in now without anything being taken apart.

yes. Does it go over the metal part or just stay on one side?

Just stick it over the metal in between the cusions. It should snap into place.