repairlarge crack in fiberglass rear cap/bumper

From the back (inside) of the damaged area you need to apply fiberglass cloth or tape and resin to restore strength to the area.
Then, on the outside, grind away the damaged surface cracks and loose sections and fill with epoxy paste (MarineTex), sand that smooth, and paint.
Getting to the back may be hard. Applying the tape and resin is easy. The sanding is tough: MarineTex is very hard.
Restoring it to like new will take someone who has done this many times.