I do not believe that the rear cowl is "fiberglass" ("fiberglass" being a generic term for glass reinforced polyester resin and mostly a fabrication process) but rather an injection-molded part. Though glass fiber is used extensively for structural reinforcement in injection-molded parts, molded in all types of plastics, it is not typically used for parts requiring a good surface finish such as the cowling. The part is most likely what they call a "structural foam" (process of molding) part, which can be molded in many different plastics but is either a high impact ABS or Polycarbonate. They inject this plastic into a mold with a thick wall/cavity and then cycle an inert gas into the cavity so that it generates a core of bubbles through the walls of the part. These bubbles create a webbing of plastic that gives you the structural walls, but when it cracks the bubbles (voids) make it difficult for repairs as it not a solid surface to solvent bond together. You should be able to determine the type of plastic that the part is made from by removing the part and looking on the back/inside surface for mold codes, which typically will convey the mold cavity number, date-molded wheel and material (PVC, PP, PC, ABS, PE, PET, etc.) With this information you should be able to determine the solvent that will work to repair the crack. If you are not sure, post the codes and I will see if I can help you decipher the information.
