Slide out trim molding exterior

I have a 1996 Overland with a slide out that has a 1 3/4" plastic molding all the around the exterior edge to cover the space between the slide out and the the side of the motor home.

I am trying to reinstall it. The edge of the molding slides into a groove with a string that is in the groove and is one continuous piece. The string is slid in with the molding I assume to wedge the molding into place. I can't get it to stay. I am working on the bottom piece and it falls out.

Can someone tell me how to install this properly.

Also I am trying to purchase some. Where can I find it.

Thanks