I'm curious as to were you got the 201/2 measurement. In the center? What is the measurement at 18' from center? That's a long time to stuffed in a box. I have stapled them to the top of my garage door and hung some weights to bottom and let them hang in the hot sun for a couple days. If rear window is scuffed or stained or mildewed you need to replace it. any auto trim shop should be able to order you what you need.