Try lacquer thinner. Soak the fiberglass with lacquer thinner, let it work for a few minutes then use a nylon scrub pad to loosen the remaining stain. You will have to apply the lacquer thinner often due to evaporation. If some stain is still left in the wood grained areas use a small nylon scrub brush to work these areas. After the stain has dissolved simply wipe it off.
The lacquer thinner will evaporate quickly so working in sections is a good idea. Don't worry if a little stain is remaining in the wood grained area. This will be covered up by your new stain.