I'm aware of two different types of these, a small central moulding and a larger full-width type. But both seem to be attached in the same way; during assembly the moulding would be placed over the fabric, with plastic pegs protruding through holes in the panel. I take it that the pegs would then be heated to deform and widen them in a similar way to a metal rivet.
Sorry to be the bearer of bad news.