I would drill a pilot hole through the plastic to see if there is a space between the plastic and the metal behind it.

When I added an R model bumper to one of our 850s, I found holes already made in the metal part. I did some careful measuring.

What type of expansion bolts are you using?

Most are meant to be a snug fit into the hole that you drill.

The square surfaced ones for older bumpers have a smaller portion near the surface so that they snap into a thin bumper.

The cone-shaped plastic ones with ribs fit so snug into the proper hole that they must be tapped in with a hammer.

Mollys are made to fit a certain thickness of material, I just bought some to use on a luan door - quite thin.

You could experiment with a block of wood to get a good fit.

You could also install the bolt you intend to use into the expansion fitting to see what it does when you install the bolt.