will fiberglass resin melt plastic? looking to reinforce the baja plastic side pocket, and was thinking I would just start layering fiberglass cloth... want to do something similar to below, but that plasctic pocket is really flimsy and even with the stock stereo you can hear the plastic vibrating...

Depends on the plastic and the thickness. It wont completely melt or disintegrate it, but mix resin a plastic cup and see what happens. For what you want to do, if you lay up the 1st layer with a standard resin/catalyst mix and let it cure, you should have no problem with subsequent layering.

It may not stick to the plastic

It probably only cause it to fog over. Find some ethylene dichloride, local Westlake or a hobby store. That is what is used to glue plexiglass together. It will leave a invisible seam where the 2 pieces meet. It will discolor the plexi if you get it on the sheet by itself. The way we used to do it was take the 2 edges you want to glue together, clamp in place and use a small solder rosin type bottle with a needle size hollow tube, also available from hobby store, run a bead of the e/d/cloride down the seam. It will wick in and set up almost immediately. For a stronger bond, drill and tap for flat head screws and you will have a permanent piece. Used this stuff in thermal ovens for hot and cold testing and it is pretty solid.

no, it will melt to it! I kid I kid

Ayuh,....... Use Epoxy, rather than poly resin,..... No problem,.....