I would like to be able to fix this trim piece, they are hard to find. Would fiberglass work to back it up? I don't mind if the crack shows but I will be painting the part to match my blue interior so I could fill it with a little caulk or something. Have any of you done this before or have any helpful tips?

 

3M has flexable parts repaire two part adhesive, it's not cheap but it works! There are other products that are similar that work also. Try a body shop supply jobber, some ORilleys are.